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======================= COMPTEST 2.59 ============================ Release date: June 9, 1993 COMPTEST is a program that determines the system configuration and performance characteristics of PC compatible computers. COMPTEST was designed to be fast, so most parameters are determined during program start-up and the first page of results will come up almost immediately. Even for slow systems like the original IBM PC the first page will be displayed within a few seconds. There are at most three pages with results displayed sequentially, with some tests occurring only when the appropriate page is displayed. Usage: COMPTEST [file name] [/D] [/H] [file name] is an optional parameter specifying a file in which the results displayed by COMPTEST will be saved upon termination of COMPTEST. /D is an optional switch enabling additional messages that aid in debugging COMPTEST if the program should crash or fail to correctly determine the system configuration. /H is a switch that prints a short help screen for COMPTEST. COMPTEST 2.59 is Copyright (c) 1988-1993 by Norbert Juffa Wielandtstr. 14 7500 Karlsruhe 1 Germany COMPTEST 2.59 is public domain software and is distributed with full assembly language and Turbo Pascal 6.0 source code. You are free to incorporate parts of the code into your own programs as long as you don't use it in a commercial product. Please do others a favor and always distribute the complete COMPTEST package, not only the binary. If you want to notify me of bugs you discovered in COMPTEST or want to comment on the program in any way, you can either contact me at the address above or on Internet as JUFFA@IRA.UKA.DE Revision history: Changes since version 2.58 o Detection method for Cyrix 486DCL/SLC has been changed back to method used in COMPTEST 2.57. The detection method used in version 2.58 that was based on checking the value in the destination register of a BSF instruction after executing it with a source register containing zero seems to work only with very old versions of the Intel 80486. Newer versions of the Intel 486DX/SX show exactly the same behavior as the 486DLC/SLC. Therefore, COMPTEST 2.58 reported most 486SX as 486DLC and 486DX as RapidCAD. Thanks to Jian Liu and David Ruggiero for reporting this bug. o Experimental support to detect an Intel Pentium CPU has been added. Detection is based on incomplete information, so Pentium detection and measurements may not work correctly yet. Changes since version 2.57: o Detection method for Cyrix 486DLC/SLC has been changed. The new method does not rely on timing anymore. o Timing routine has been enhanced to work more reliable with fast machines. o Documentation file have been enhanced and updated. Minor cosmetic changes to COMPTEST and MEMMAP programs. o New version of Turbo Pascal 6.0 run time library included. Changes since version 2.56: o COMPTEST 2.56 was compiled with the wrong library. Therefore, benchmark results for the floating-point benchmarks (Whetstone, LLL) differ from other versions of the program. Sorry about the mistake! o A correction was added to the determination of coprocessor clock frequency in the case a Cyrix 486DLC CPU is present. With a 486DLC present, the block of FSQRTs that determines the coprocessor clock frequency is executed faster than with the Intel 386DX as CPU, due to the improved communication between CPU and coprocessor. The observed speedup is in range between 4.7% and 9.7%. For simplicity, COMPTEST 2.59 uses a simple correction that divides the computed frequency by 1.055. Changes since version 2.55: o COMPTEST 2.55 wouldn't detect a Cyrix EMC87 if it was installed and reported it as a Cyrix 83D87 instead. This has been fixed. o Correct detection of the presence and clock frequency of the Cyrix 486DLC has been tested. Note that presence of a Cyrix 486DLC may lead to a higher clock frequency being displayed for a coprocessor, if one is installed. Since the 486DLC has an improved handshaking with the coprocessor, the block of FSQRT instructions used to measure the coprocessor's clock is executed up to 10% faster and the reported clock frequency of the coprocessor goes up accordingly. Changes since version 2.54: o Enhanced POPAD bug detection by testing POPAD execution using 31 different initial values in the EAX register. Previously, only one value was used, which made correct detection of the bug somewhat unreliable. o Detection of the SuperMath 38700DX and 38700SX coprocessors from Chips&Technologies have been added and successfully tested. Detection for the Intel RapidCAD, Intel 387DX, and Cyrix 387+ has been changed from tests depending on instruction timing to tests that rely on certain small incompatibilities between the coprocessors. o When called with a filename to store the results in, COMPTEST would fail to print the final message "COMPTEST terminated - press any key" if either there was a error in handling the file indicated or if no hard disk results were stored. This has been fixed. ====================================================================== The following is a detailed explanation of the information provided by COMPTEST 2.59 and the known limitations of the program. o General limitations: Correct execution of COMPTEST depends on certain hardware resources provided by the hardware of PC compatibles. That a machine runs MS-DOS is by itself no guarantee that COMPTEST can be run successfully, as MS-DOS may run on machines that are not 100% compatible with industry standard PCs. Since it directly accesses hardware components, COMPTEST should not be run in the DOS-boxes supplied by Windows, OS/2 or similar programs. Even if it does run, some or all of the information it provides may be wrong or misleading. For the same reason, it should not be run on a PC emulator, e.g. SoftPC by Insignia, even if simpler programs like the Landmark Speed Test run successfully in such an environment. o Computer type: The specific type of an IBM compatible PC is coded into one or two bytes in the last 16 bytes of the first MByte of the address range. This memory is occupied by the BIOS ROM. Quiet a few values have been defined by IBM for its PC, AT, and PS/2 computers. COMPTEST decodes this information according to IBM's definitions and prints the result. Ordinary 286, 386, and 486 based PCs with an ISA bus are reported as AT compatibles. o CPU Type: COMPTEST is able to detect the following CPU types: Intel 8088, Intel 8086, NEC V20, NEC V30, Intel 80188, Intel 80186, Intel 80286, Intel 80386DX, Intel 80386SX, Intel 80486DX, Intel 80486SX, Intel RapidCAD, Chips&Technologies 38600DX, Cyrix 486DLC, Cyrix 486SLC. The Intel 80186/80188 are CPUs with integrated peripherals that have been used in only a few PCs that were manufactured around 1982/1983. It has an extended 8086 instruction set. The Intel RapidCAD is a replacement for a 80386/80387 combination and is basically a 80486DX without the internal cache and with a 386 pinout. The Chips&Technologies 38600DX is a pin compatible replacement for the 80386DX CPU that offers some performance improvements. The Intel 486SX is a 80486 without the FPU (floating point unit). The Cyrix 486DLC is a CPU that is software compatible with the 486SX, but is a replacement for the 80386DX CPU. Similarly, the Cyrix 486SLC is for use in 386SX systems. AMD's Am386DX, Am38DXL, Am386SX, and Am386SXL are 100% compatible to the Intel 386DX and Intel 386SX CPUs, respectively, and are reported as Intel 386DX and Intel 386SX, respectively, by COMPTEST. The Intel 486DX2-50, Intel 486DX2-66, and the Intel Overdrive processors are 486DX chips that use a clock-doubler that drives the CPU internally at twice the speed of all other system components. The 486DX2-50 is a replacement for the 486DX-25, while the 486DX2-66 replaces the 486DX-33. The Overdrive goes into the 487SX socket found in many 486SX systems. These processors are all reported as 80486 processors by COMPTEST. To distinguish between the different CPUs and math coprocessors, COMPTEST in most cases takes advantage of certain incompatibilities between the chips that can be tested without using other system resources. Only a few tests involve timing differences in the execution of certain instructions. These depend on the correct operation of the PC's timer chip. To separate the 8086, 8088, V20, V30, 80186, and 80188 CPU from newer CPUs, the behavior of the instruction sequence PUSH SP, POP AX is used. While after the execution of these instructions, AX=SP on the newer processors, AX=SP-2 for the 8086..80188. To distinguish among the CPUs in the first group (8086..80188), the following properties of the processors are used. The 80186/ 80188 (like all newer Intel CPUs) mask off shift counts MOD 32 in shift and rotate instructions so that no more than 31 shift steps are performed. The V20, V30, 8088, and 8008 do not mask off shift counts and perform up to the 255 shift steps allowed by the 8-bit counter used. If a register with a non-zero contents is shifted left with a shift count of 32, it will be cleared after the operation on the 8086/8088 and V20/V30, while nothing will happen on the 80186/80188, since 32 MOD 32 = 0, that is, no shifting takes place. V20 and V30 (just like the 80186/80188 and newer CPUs from Intel) have a PUSHA instruction that saves all general registers on the stack. The 8086/8088 don't have this instruction, but the execution of the PUSHA opcode acts like a JMP skipping the next code byte. COMPTEST uses this code byte to set a flag so that the flag is only set on V20/V30 processors. If the flag is not set, an 8086/8086 must be present. This is verified by an additional test that checks if the highest nibble in the flag register can be set. This nibble is always cleared on the 8086/8088 and can not be set. The 8086, V30, and 80186, which have a 16-bit data bus, can be distinguished from the 8088, V20, and 80188, which have an 8-bit data bus through the use of self modifying code that works differently due to the different length of the instruction prefetch queue, which has a length of 4 bytes for the CPUs with 8-bit busses, but a length of 6 bytes for the CPUs with 16-bit busses. Modifying an instruction five bytes ahead in the instruction stream will cause the modified instruction to be executed by the 8-bit CPU versions, while the original instruction will be executed on the 16-bit CPU versions, since it was already in the prefetch queue by the time the instruction was modified in memory. To tell apart the 80286 from the 80386 and 80486, an attempt is made to change certain bits in the flag register of the CPU. While they can be modified in the 80386 and 80486, the 80286 will not allow that to be done. The 80486 has a new bit in its flag register that is not defined in the 80386 and is always clear there. By attempting to toggle this bit, it can be decided whether a 80386 or 80486 is present. The 486SX is a 80486 without the FPU (floating point unit ~ integrated coprocessor), so if a 486 CPU has been detected but the test for a coprocessor or FPU fails, it can be concluded that a 486SX is present. The 80386SX has a 16-bit data bus as compared to the 32-bit data bus of the otherwise (almost) identical 80386DX, so 32-bit memory accesses on the 80386SX are slower than 16-bit memory accesses since they have to be split into two 16-bit accesses. On the 386DX, both 16-bit and 32-bit memory accesses have the same speed, if memory operands on addresses divisible by four are accessed. By measuring and comparing the speed of 16-bit and 32-bit memory accesses, COMPTEST determines if a 386SX is present. Intel and AMD both make 386DX and 386SX processors that are functionally totally identical. However, AMD makes 386DXs that are rated for 40 MHz and 386SXs that are rated for 33 MHz, which Intel doesn't make. So COMPTEST could make an educated guess on what manufacturer's CPU is used based on the clock frequency it determines. COMPTEST does without this guess, though, and reports all AMD 386 CPUs as Intel 386DX or Intel 386SX. Chips and Technologies has introduced CPUs that are compatible with the 386DX and 386SX, which are called the 83600DX and 83600SX. Also, an 83600DX with a small internal cache has been announced called the 83605DX. While AMD uses Intel's microcode in their 386 CPUs, C&T uses its own microcode. Therefore, the CPUs from C&T do not possess a well known bug present in the Intel 80386. This so called POPAD bug causes the EAX register to be trashed for a certain instruction sequence involving the POPAD instruction. COMPTEST checks for this bug to distinguish the C&T CPUs from Intel's 386 processors. Since I have found that the POPAD bug can not be reliably reproduced on Intel 386SX CPUs, COMPTEST reports all 386SX CPUs as Intel 386SX, whether a POPAD failure occurs or not. Since C&T will not offer the 38600SX before late in 1993, this doesn't make the CPU detection by COMPTEST less reliable. COMPTEST will not recognize the 83605DX, but will report it as an 83600DX. Since the reproducibility of the POPAD bug depends somewhat on the initial value of the EAX register, COMPTEST uses 31 different values. The Intel RapidCAD is basically a 486 without the internal cache that is an end user replacement for a 80386DX/80387 combination. It is 100% software compatible with this combination and can be detected by checking the speed of store operations from the FPU to memory, which are executed much faster on the RapidCAD than in any 386/387 system, regardless of the coprocessor used. Cyrix now offers the 486DLC and the 486SLC that are designed for 386/386SX systems. However, they are software compatible with the Intel 486SX. Cyrix has implemented a fast array multiplier on these chips to speed up integer multiplications, making the MUL instruction faster than on any other CPU found in PC compatible computers. COMPTEST detects the Cyrix 486DLC/SLC by comparing the speed of the MUL and AAM instructions. On the 80486, the execution time for the MUL instruction ranges from 13 to 26 clock cycles for a 16 bit operand, while the AAM instruction executes in 15 clock cycles. So multiplication is never significantly faster than AAM. On the Cyrix processors, MUL takes 3 cycles with a 16 bit operand, and AAM takes 16 cylces. So on these processors MUL is several times faster than AAM. o Clock frequency: Measuring the clock frequency of the CPU is based on repeated execution of the AAM (ASCII adjust after multiply) instruction. This instruction takes more than 10 clock cycles to execute on all CPUs that are supported, so there is enough time for the CPU to always keep its prefetch queue filled, resulting in very stable timings since there is no additional penalty for filling up the prefetch queue. Also the AAM instruction has the advantage to execute in a fixed number of clock cycles, as opposed to some instructions like DIV that take even longer to execute than AAM, but whose timing depends on the input arguments. To report the clock frequency accurately, it is absolutely important to use the correct execution time for the AAM instruction in COMPTEST. Note that the AAM execution time stated in the Intel manual for the 8088/8086 is not correct. Depending on the accuracy of the oscillator that drives the PCs timer, the reported CPU clock frequency should be accurate to within +/- 2%. Note that for CPUs that use internal clock doubler circuits (e.g. Intel 486DX2-50), the clock frequency displayed by COMPTEST is the frequency at which the CPU runs internally (50 MHz in the example cited). o Bus width: The width of the CPU data bus. This is determined by the type of the CPU, so this is actually redundant information. o Cache size: COMPTEST is one of very few test programs that correctly determine the cache size of first and second level CPU caches. This is very useful if you are not sure whether the CPU cache in your computer is enabled or functional at all. COMPTEST moves memory blocks of increasing size and watches for sharp drops in memory throughput to determine the cache sizes. Since the largest blocks tested have a size of 512 KB, COMPTEST is limited in that it can *not* correctly detect CPU caches that are bigger than 256 KB. The smallest cache size COMPTEST can determine is 1 KB, which is the size of the internal cache on the Cyrix 486SLC and 486DLC chips. COMPTEST's cache test strategy may be defeated if you have defined non-cacheable areas in the first 512 KB of base memory. Non-cacheable areas can usually be defined in the extended BIOS setups of 386 and 486 based machines and are only necessary if you have a write-back cache and have to ensure correct operation of memory mapped peripherals (e.g. video memory of graphics card, Weitek coprocessor). Usually there is no need to define a non-cacheable area in the first 512 KB of base memory. COMPTEST's cache detection usually is very reliable, only once did it indicate a cache on a system with no CPU cache, probably due to the mixture of page/interleave access modes used by that system's memory. The technique used by COMPTEST to determine cache size basically works as follows: Assume you have a machine with a n KB CPU cache. If you read a memory block of n KB or less twice, it will be read almost completely from the cache the second time it is read (some data may have been thrown out by accesses to the code of the test program, as the caches in the Intel 486 and comptible CPUs is a unified code and data cache). However, if you linearly read a block of 2n KB data twice, the second half of the data block will throw out the first half of the data block that is already in the cache. On the second pass through, accesses to the first half of the block will result in a miss for every cache line accessed, as the cache now contains the data from the first n KBytes of the block. If the times to read data blocks of 2^i KBytes are recorded, one sees a sharp increase in read time as soon as a data block larger than the cache size is read. COMPTEST uses block moves instead of the block reads in the example, as I have found this to be somewhat more reliable. o Maximum RAM throughput (without cache): This is a measure of the quality of the main memory system of the machine tested. As with all throughput numbers, higher numbers are better. Maximum throughput is determined by executing block move instructions moving blocks on addresses divisble by four. For processors up to and including the 80286, 16 bit transfers are used to move the data. For the 80386 and newer processors, 32 bit transfers are used to correctly reflect the higher memory throughput that is possible using instructions that can handle 32 bit data. By moving memory blocks that are bigger than CPU caches that may be present, COMPTEST tries to defeat the cache strategy and to measure the true speed of system RAM as if no cache(s) were present. However, this technique can back- fire so the values given by COMPTEST for RAM throughput without cache may be different from those that COMPTEST determines if the cache(s) are physically disabled (usually possible through the BIOS setup). The value reported by COMPTEST for RAM throughput without cache is really the memory throughput with the maximum possible number of cache misses. With a decent cache controller, the memory access speed in case of a cache miss is the same as if no cache were present at all. However, some cache controllers impose an additional overhead on such a memory access that may be as large as 40 clock cycles per cache line loaded, as opposed to 3-4 clocks for a good cache controller. In these cases, COMPTEST reports up to 6 wait states or even more for RAM access without cache. Even if COMPTEST does not report the correct value for the RAM throughput in these cases, it is still a valuable indicator of the quality of the cache/memory subsystem implementation. The higher the throughput reported the better does the system perform. On systems with no CPU cache, COMPTEST reliably measures the true throughput of the system RAM. Based on the measured throughput as compared to the maximum throughput for the detected CPU, COMPTEST also computes the equivalent number of wait states. This is not an integral number due to the fact that the number of wait states is usually not the same for every memory access. COMPTEST reports the *average* number of wait states needed. With wait states, lower numbers are better. Older 80286 based systems going faster than 10 MHz usually have at least 0.6-0.7 wait states but on a recently tested 80286 system running at 16 MHz, COMPTEST reported 0.1 wait states, which reflects the state of the art in memory system design. 80386DX and 80486 based systems typically have no less than 1.6 wait states. A brand-new 386SX system based on the Am386SX-33 had only 0.3 wait states, though. There are machines that use fast SRAM for the 640 KB base memory that doesn't force the CPU to insert wait states when accessing this memory. Please note that systems using clock doubled chips will often report more wait states than systems in which the CPU runs at the same speed as the other system components including the memory subsystem. In systems with a clock doubled CPU, memory always looks slow to the CPU, as one clock cycle on the memory data bus equals two internal CPU clock cycles. Since COMPTEST reports waits states measured in CPU clock cycles, one will see the wait states reported double when changing from a 486DX-33 to a 486DX2-66 on the same motherboard. For example, a board for which COMPTEST reported 2.6 wait states when run with a 486-33 CPU will have 5.2 wait states reported after changing to a 486DX2-66 CPU. Reporting the wait states measured in internal CPU clock cycles is well justified, as the number of wait states tell the user something about the relative speed of the memory compared to the speed of the CPU. Clearly, a memory subsystem running at 33 MHz is more adequate for a CPU running at 33 MHz than for one running at 66 MHz. The RAM throughput measured by COMPTEST refers only to base memory (first 640 KB of memory or less). o Cache Throughput: This information is only reported if COMPTEST has found a CPU cache in the machine. As with memory throughput, higher numbers are better. Cache throughput is determined by performing block moves on addresses divisible by four within the cache memory using the CPU's MOVS instruction with the maximum data width available. If two levels of cache are present in the system (e.g. a 80486 system with an external cache of 256 KB and the 8 KB of internal cache in the 80486), the throughput for both is reported. You will see a performance drop going down the memory hierarchy. First level caches usually run with no wait states, that is with the full speed supported by the CPU. The second level cache has less throughput than the first level cache, but is several times larger. The system memory is even slower than the second level cache but much bigger. The cache throughput rates reported by COMPTEST usually are accurate indicators of the cache performance. Write-back caches may have higher throughput rates reported than write-through caches, as the block move performs reads and writes. However, write-back caches also show higher performance when used with real applications, so the higher performance indicated by COMPTEST is probably justified. o System memory: System memory here refers to the base memory within the first megabyte of the address space and below the start of a graphics adapter's display memory. COMPTEST searches for RAM in small steps from the bottom to the top of the address space until it reaches a graphics adapter or no more contiguous RAM is found. Note that this value can be larger than the usual 640 KB. For example, on systems that have only a CGA, system memory could be expanded up to address B8000h for a total of 736 KBytes of system memory. Similarly, system memory could be expanded to 704 KBytes on a system using a monochrome Hercules card. There are special memory cards that allow such extensions. Also, utilities such as the VIDRAM program included with Quarterdeck's QEMM memory manager can expand the base memory by either using part of a VGA's or EGA's display memory as system memory or mapping extended memory into this range, and disabling part of the EGA/VGA's capabilities to make basically a CGA out of them. o Memory available to DOS: This is the amount of system memory (base memory) that the BIOS reports to DOS. The BIOS determines the amount of system memory during a cold boot and stores the result in its data block which starts at address 400h. Several utility programs that extend the system memory above 640 KB, such as the VIDRAM program that comes with Quarterdeck's QEMM memory manager, manipulate this value to reflect the greater amount of memory now available to DOS. Note that the value reported by COMPTEST does not include DOS memory in UMBs created by MS-DOS 5.0 or similar programs. This test is a bit out of date and could be updated to support the new features of the latest DOS versions. o Memory permanently used by DOS and TSRs: As the previous test, this one is outdated in that it doesn't take into consideration the state of the art with regard to DOS memory management. All memory below the address at which COMPTEST is loaded is assumed to be unavailable to DOS. Device drivers and TSRs that are loaded high are not included into the amount of memory reported. Use the program MEMMAP also included in the COMPTEST archive file to get a detailed list of memory blocks allocated to DOS and TSRs. o Extended memory (INT 15h throughput): Extended memory is that part of a PC's memory that is above the first megabyte of the CPU's address space. It can be available only on those computers that have an 80286 or newer CPU. Except for the first 64 KB block of extended memory, which is called HMA (high memory area), it can only be accessed in the protected mode of these processors. COMPTEST reports the amount of extended memory as determined by the system BIOS during a cold boot. This value is stored in the CMOS RAM of the real time clock of AT type machines. On newer PCs, this CMOS has been physically incorporated into the chip set that contains most of the discrete logic of older PCs in two or three chips. COMPTEST reads the amount of extended memory directly from the CMOS RAM. Note that the sum of base memory and extended memory may not add up to the total memory installed in the machine, as some of this memory may be used to shadow the BIOS ROM and/or ROM extensions (e.g. VGA BIOS) and is therefore unavailable for other purposes. Shadowing means that the code in the ROMs is copied to RAM during a cold boot and that this RAM is mapped to the same address as the shadowed ROM. Since ROMs are slow, code in the ROMs (e.g. BIOS) executes slowly and can be sped up a lot by shadowing. Extended memory can be accessed in several ways. One way is to use the services of a XMS (extended memory specification) driver such as HIMEM.SYS. This, however, requires such a driver to be loaded. There are also functions provided by the BIOS via INT 15h to access extended memory. COMPTEST uses these to copy a block of memory from extended memory to the base memory below 640 KB and determines the transfer rate (throughput) by measuring the time it takes to copy the block. Using a memory manager such as QEMM usually causes the INT 15h functions to be mapped to the appropriate XMS calls, so the INT 15h throughput value may differ significantly depending on whether a memory manager is loaded or not. Not all BIOSes use 32-bit transfers for block copies from/to extended memory when it is possible (that is, on 386 and 486 based machines), so the INT 15h throughput from extended memory may be only half of the normal system memory throughput. Also, access to extended memory usually requires the CPU to be switched to protected mode and back, which causes considerable overhead when it is not done via the fast methods provided by most 386 and 486 chip sets, but uses the traditional method which involves using the keyboard controller. o Expanded Memory: Expanded Memory, specified in the EMS (expanded memory specification), was originally designed to provide 8086 and 8088 based computers, which have only a one MByte address space, with up to 32 MBytes of memory. The LIM (Lotus, Intel, Microsoft)-EMS is a standardized application interface that permits several implementation techniques. Memory cards which support expanded memory in hardware use sort of a bank switching technique. Up to four blocks of 16 KBytes each can be mapped into a contiguous 64 KB region in the address range C8000h-E0000h. This region is called the EMS page frame. The memory on such EMS cards can not be accessed as fast as the system memory on the motherboard in most computers, since the data has to travel over the relatively slow ISA bus. On the 386 and 486 based computers mostly used nowadays, expanded memory is usually provided by a memory manager like 386MAX, QEMM, or EMM386 that manages part of the extended memory (memory above the first MByte of the address space) as expanded memory. These programs use the MMU (memory management unit) built into these CPUs to map memory blocks from the extended memory to the EMS page frame. There are also programs that use the hard disk to provide the storage for expanded memory. COMPTEST tries to detect an EMS driver in the system. If it finds one, it will question the driver for the total EMS memory provided by it, the start address of the EMS page frame and the EMS version number with which the EMS driver complies. The current version of EMS is 4.0, which defines additional services over the previous version 3.2. COMPTEST does not detect a memory card with hardware support for EMS if the EMS driver for the card has not been loaded. COMPTEST determines the throughput from EMS memory by reserving a EMS-page, mapping it into the page frame and doing a block copy from the mapped-in page. Note that the total amount of memory available to programs that can make use of expanded and extended memory may well be lower than the sum of the extended memory and the expanded memory, as some of the extended memory physically present in the machine and reported by COMPTEST may have been logically converted to EMS memory by an EMS driver. For example, the QEMM 6.0 memory manager provides extended memory according to XMS and expanded memory via a built-in EMS driver, and satisfies memory allocation request from a common pool for both types of memory. So for a machine with 8 MBytes of physical memory it may report 7 MBytes of EMS and 7 Mbytes of extended memory. o other RAM: COMPTEST tries to find additional RAM between the end of the graphics card's display memory and the start of the BIOS ROM. This RAM may be provided by special memory cards or by some chips sets like NEAT that can map memory to this region physically. It can also be provided by 386 memory managers, that use the processor's MMU (memory management unit) to logically map memory to this region. The latest DOS versions (e.g. MS-DOS 5.0) can use such memory in the form of UMBs (upper memory blocks). COMPTEST may also find RAM that is present on network adapters or certain hard disk controllers. In such cases, COMPTEST may report numerous very short RAM blocks. If the length of these blocks is below one KByte, COMPTEST prints the size as 0 KB. The memory found on network adapters and hard disk controllers is of course not available to DOS. Rather, these adapters use the RAM for buffers or to hold certain variables. COMPTEST does not scan the address space above the display memory byte by byte to find RAM. Rather it tests every 256th byte if it is in RAM. The byte is tested by writing two different values to it and checking if both can be reliably read back. o BIOS-extensions: COMPTEST searches for BIOS-extensions such as a VGA-BIOS or hard disk BIOS between the end of the graphic adapter's display memory and the start of the main BIOS-ROM. COMPTEST checks in steps of 256 bytes if the next two bytes read 55h, AAh, which is the common ID with which all BIOS extensions start. If the sequence 55h, AAh is found, COMPTEST reads the next byte, which stores the length of the BIOS-ROM measured in 0.5 KByte blocks, if a BIOS-extension is indeed present. All bytes in the memory region specified by the length byte are summed up. If the 8 lowest bits of the sum are found to be all zero, a valid BIOS extension has been found. COMPTEST then tries to determine if the BIOS extension found is a hard disk BIOS or an EGA/VGA BIOS and displays that additional information where applicable. Note that the same block of memory can be displayed as both, a BIOS extension and extra RAM. This usually indicates that BIOS shadowing is being used and that the shadowed BIOS has not been write protected. o parallel ports: COMPTEST prints the number of parallel ports as reported in the BIOS's equipment byte. o serial ports: PCs are commonly prepared to manage up to four serial ports in the system. The BIOS checks for serial ports installed during a cold boot and stores the number of serial ports found in the BIOS' data area (starting at address 400h). It also determines the start address for the block of IO-ports each serial port occupies. COMPTEST evaluates this information. It also tests for serial ports (UARTs = universal asynchronous receiver transmitter) itself, searching the four standardized IO starting addresses used in PCs (3F8h, 2F8h, 3E8h, 2E8h). First it tries to establish whether a UART is present at the specified address by trying to switch on the loop back feature of the UART and then transferring a byte through the loop. If this test passes, COMPTEST assumes that a UART is present, since it is highly unlikely that any other device would correspond to the same sequence of instructions in the same manner. COMPTEST then tries to find out whether the UART chip used is a 8250, 16450, 16550, or 16550A. The 8250 is the UART chip used in PC compatibles. The 16450 is the successor to the 8250 chip. It supports transfers at higher baud rates than the 8250. It also features a scratch register that the 8250 does not have. By trying to store values in the scratch register and read them back, COMPTEST determines whether a 8250 or 16450 is in the system. The 82450 is the same chip as the 16450 produced by a different manufacturer and is reported as a 16450 by COMPTEST. The 16550 is a 16450 with added send and receive FIFO buffers. This makes for more reliable communication and higher effective transfer rates in interrupt driven serial communication. The original 16550 had a bug that was fixed in the 16550A. The 16550 has two status bits that reflect the status of the send/receive FIFOs. On the 16550 only one of these bits works correctly, while on the 16550A, both of them perform as expected. This is used by COMPTEST to distinguish between the two chips. o mathematical coprocessor: COMPTEST checks if a 80x87 mathematical coprocessor is present. If one is found, it does a detailed check on the type of coprocessor installed. It can determine the presence of the Intel 8087, Intel 80187, Intel 80287, Intel 287XL, Intel 80387, Intel 387DX, Intel 387SX, Intel RapidCAD, Cyrix 82S87, Cyrix 83S87, Cyrix 83D87, Cyrix 387+, Cyrix EMC87, IIT 2C87, IIT 3C87, IIT 3C87SX, ULSI 83S87, ULSI 83C87, C&T 38700DX, C&T 38700SX and coprocessor emulators using INT 7 for emulation. The Cyrix EMC87 is a 387DX compatible coprocessor that also provides a memory mapped mode and goes into the EMC socket (found in most 386 based machines) that was originally designed for the Weitek coprocessors. Correct detection of most, but not all, of the chips mentioned has been tested. COMPTEST also tries to determine the presence of a Weitek Abacus 3167 or 4167 coprocessor by checking the BIOS' equipment status for a set Weitek bit. However, on most systems, this bit is only set if the Weitek coprocessor has been registered in the BIOS extended setup by the user, so it is not very reliable. COMPTEST does not check for physical presence of a Weitek coprocessor. COMPTEST takes advantage of the fact that 80x87 coprocessor instructions are ignored in systems with no coprocessor. It executes instructions that store the default status and control words of a coprocessor to memory. If no coprocessor is present, nothing gets stored in these memory locations. If the expected values are stored in these locations COMPTEST knows that a coprocessor is present. If a coprocessor has been found, COMPTEST tries to detect into which of the following four groups it belongs: emulator via INT 7, 80486, 8087/80287, all other coprocessors. If the emulation bit in the machine status word of a 286 or 386 CPU is set, COMPTEST assumes that the 'coprocessor' found is actually an emulator that emulates coprocessor instructions via the INT 7 trap. If the CPU was found to be an Intel 80486, COMPTEST knows that the coprocessor found is the FPU on this chip. The Intel 8087 and 80287 were designed before the IEEE-754 Standard for Binary Floating-Point Arithmetic was finally accepted in 1985. As opposed to all newer coprocessors, they implemented certain features no longer supported in the final form of the standard. One of these features was that they had two modes for handling infinities. In one mode, infinities were signed, in the other, all infinities did not carry a sign and were the same. COMPTEST uses this to separate the 8087 and 80287 from other coprocessors. It generates an infinity by means of a division by zero, duplicates that infinity, changes the sign of the infinity and then compared the two values. On the 8087 and 80287, they will be reported to be identical, while on all other coprocessors, the are reported as different due to the different sign. This enables COMPTEST to distinguish the Intel 80287 from the 287XL and coprocessors compatible with the latter, such as Cyrix 82S87 and IIT 2C87. It also makes it possible to find out whether a 386 based system uses the 287 or a 387 as the coprocessor. The 287 and 387 compatible coprocessors from different manufacturers can be told apart by certain incompatibilities: The IIT coprocessors do not support denormal numbers in the coprocessor's internal format, while all other coprocessors do. COMPTEST tests for the IIT coprocessors by loading an extended precision denormal and adding that number to itself. On all coprocessors except the ones from IIT, this causes the denormal exception to be raised. Since the result is flushed to zero on the IIT coprocessors, the denormal exception is not raised. The ULSI coprocessors do not support the rounding control feature of the other coprocessors. They compute all results in extended precision. To test for ULSI coprocessors, COMPTEST sets the precision control to 53 bits and then multiplies two numbers whose product can be represented exactly in the 64 mantissa bits of the extended precision format, but not in 53 mantissa bits. Therefore, the precision exception is raised on all coprocessors except the ones from ULSI. In the Cyrix coprocessors, several small bugs present in the Intel coprocessors have been fixed. One of them deals with the operation on NaNs. Intel's requirements state that an instruction that operates on two NaNs should return the larger of the two NaNs. However, if both NaNs have the same absolute value but different sign, Intel's coprocessors erroneously return the negative (and therefore smaller) NaN. The Cyrix coprocessors return the correct result in these cases. COMPTEST uses the FPATAN instruction to perform the test described. The successor to the 83D87 from Cyrix is the 387+. This is a "Europe-only" name, in other parts of the world, the new coprocessor is sold under the old name 83D87. The 387+ can be told apart from the 83D87 because of its extended argument range for the FYL2XP1 instruction. While the range for this instruction is restricted to -sqrt(2)/2..sqrt(2)/2 on all other 80x87 compatibles, it is unrestricted in the 387+. COMPTEST computes FYL2XP1 (1.0) and tests if the correct result (1.0) is returned. The Cyrix EMC87 can be told apart from other Cyrix coprocessors since the most significant bit of its control word can be written. The Intel 80387 exists in two versions. The newer one is called 387DX and provides about 20% more performance. One difference between these chips is what they get for the exponent when doing an FXTRACT of -1.0. While the older 80387 gets -0.0 as the answer, the newer 387DX gets +0.0. This difference is used by COMPTEST to decide which Intel 80387 is in the machine. The coprocessors from Chips and Technologies are detected by the result they return for F2XM1 (pi). Note that F2XM1 is only defined for arguments in the interval -1..1. The C&T 38700 coprocessor returns pi/2 when F2XM1 is called with an argument of pi. The Cyrix coprocessors return the same result, but are never submitted to the test for the C&T coprocessors. The Intel RapidCAD behaves like a 386/387 combination. One of the few differences is the way in which the value BCD INDEFINITE is stored. While the Intel 80387 and 387DX store it as FFFF 8000000000000000, the RapidCAD and the Intel 80486 store it as FFFF C000000000000000. This difference is used by COMPTEST to detect the RapidCAD chip. COMPTEST measures the clock speed of the coprocessor by measuring the time it takes to execute a block of FSQRT instructions. This instruction was picked since it has a very stable execution time that varies only minimally and has a sufficiently high execution time. However, the execution time of coprocessor instructions in 286 and 386 systems may vary by a few clock cycles, depending on the chip set used. Also, in some systems, the CPU and the coprocessor run asynchronously, causing the execution time of coprocessor instructions to vary even more, since in 286 and 386 systems the CPU has to fetch the instructions and operands for the coprocessor. o mouse: COMPTEST tries to detect the presence of a mouse driver, not if a mouse if physically hooked up to the PC. It calls a specific mouse driver function that returns the information which mouse button has been pressed. This has the advantage that the mouse driver's status is not changed. o games adapter: COMPTEST tries to determine the presence of a games adapter (used to hook up up to two analog joysticks to the PC). This test has two stages: In the first stage COMPTEST asks the BIOS if a games adapter is present, in the second stage it tries to access the games adapters registers. Unfortunately, both methods seem to be highly unreliable, as COMPTEST usually reports that no games adapter could be found, even if one is installed. o DOS drives: COMPTEST determines the number of DOS drives by trying to set the default drive to DOS drives 0 to 8, and returns all those drives as valid DOS drives that can be used as DOS default drive. Note that this limits the number of DOS drives that COMPTEST recognizes to a maximum of nine drives. This can easily be expanded by some changes to the source code. o floppy drives: COMPTEST reports the number of floppy drives as reported in the BIOS equipment flag. In AT compatible systems, the type of each floppy drive is taken from the drive information found in the CMOS RAM in the real time clock. In modern system, this RAM is now part of the system's chip set. o hard disks: COMPTEST recognizes up to four hard disks. For each drive, it calls the 'drive ready' status function of the BIOS. Every drive that returns the 'ready' condition is included into the final tally. Some removable hard disks, such as Tandon's data packs, that require special drivers to hook into the DOS file system, are not recognized as hard disks by COMPTEST. o graphics card: COMPTEST reports only one graphics card in the system, even if two are installed (e.g. EGA and Hercules). It recognizes MDA and CGA (found in the original IBM-PC), EGA (introduced with the IBM-AT), MCGA and VGA (introduced with IBM's PS/2), also the monochrome Hercules card and IBM's PGA. No attempt is made to distinguish between the many different chip sets used in today's VGAs (e.g. TVGA, Tseng ET4000, Video 7). COMPTEST will also not recognize the Hercules RAMFont and Hercules InColor cards, 8514/A, Tiga cards or other accelerated graphics cards. COMPTEST detects most of the graphics cards it recognizes by making call's to certain functions in their BIOS. For EGA cards, it also reports the amount of memory on the adapter as reported by the EGA-BIOS. o Video-RAM wait states: The CPU usually can not access the display memory on a graphics card at full speed due to a number of reasons. As the CRT controller on the graphics adapter has to read out the display memory to generate the CRT signals and the DRAM found on most graphics cards does not allow simultaneous access from the CPU and the CRT controller, the CPU may have to wait until the CRT controller has finished its access to a particular part of display memory. Second, data transferred to/from a graphics card has to travel over the PCs system bus, which has a limited throughput that is much smaller than the memory bandwidth of the CPU, thus slowing down the average memory access over the bus. The ISA bus found in most PCs is particularly slow, while the MCA and EISA busses provide more bandwidth. To overcome this problem, some manufacturers have chosen to integrate the graphics adapter on the mother board or couple the graphics adapter more closely to the CPU using a technique called local bus. Local busses are direct extensions of the CPU's busses. They usually run at the full speed of the system and provide high bandwidth, but can only drive a limited number of cards. A popular form of the local bus concept is the VESA local bus (VLB), for which numerous graphics cards are now available. Third, for fast machines, the speed of the DRAM chips on many graphics card (60-70 ns at best) is to slow to allow zero wait state operation of video memory accesses. This is the same problem that affects memory accesses to the system RAM in these machines. Use of VRAM eases the problem somewhat, so all fast graphics cards now use VRAM. Fourth, the bus interface used by the graphic card's chip set may introduce additional slow down due to the physical organization of the display memory (e.g. remapping word accesses to byte accesses). End users can influence the raw video throughput (and thereby the number of wait states) by selecting a graphics card with a fast chip set and by configuring their system to use as high a bus speed as possible. Typical numbers for video-RAM wait states are 1 wait state per MHz CPU clock frequency for Hercules cards, ~15 wait states for EGA cards, and as low as 7 wait states for fast VGA cards (e.g. those using the ET4000 chip set). Note that on 486-DX2 system, the number of wait states is usually higher since the whole system runs at half the speed of the CPU. To measure wait states for video-RAM accesses, COMPTEST stores a block of data to the video adapter and measures the time to do that. This time is then compared to the time it would take to store this data in zero wait state memory. From these values the number of wait states is computed. COMPTEST uses the memory at address B0000h for monochrome modes and B8000h for color modes for this test. On some graphics cards, the memory access at these addresses may have a higher number of wait states than in other parts of the screen memory (e.g. the memory at A0000h used by high resolution graphics modes of EGA/VGA cards). There is also a PD program called VIDSPEED that uses a technique similar to COMPTEST's to report video-RAM throughput and vertical and horizontal retrace frequencies. Note that for graphics cards with accelerator chips, the speed with which the CPU can access the RAM on the graphics card is not a good indicator of the Windows, OS/2, or X-Windows performance, as many operations on the cards are performed on the card itself. o Speed of video output via BIOS: The speed is given in characters output per second as measured for function #9 of the video BIOS (write character with attribute). Note that there are other video-BIOS functions that write characters to the screen, that may be faster or slower than the function to be chosen for COMPTEST. For these reasons, it is hard to compare the output speed determined by COMPTEST with other system diagnostic programs such as DiagSoft's PowerMeter. As this test heavily exercises code in the video-BIOS, there may be huge performance differences between a shadowed and a non-shadowed BIOS. BIOS shadowing means that the BIOS code is copied from slow ROMs to fast RAM for faster execution at system start up. This is an option in most 286/386/486 based systems that operate at more than 10 MHz. BIOS throughput drops due to the use of the 386 memory managers, since these programs intercept all interrupts and therefore introduce a considerable overhead into the execution of BIOS interrupts. There may also be TSRs that hook the video BIOS interrupt and cause BIOS throughput to drop even further. The typical BIOS throughput in fast 386 and 486 systems usually exceeds 100,000 characters per second. Due to the trend to GUIs (graphical user interfaces), the output speed of the BIOS has lost its earlier importance. Most programs do not even use the BIOS for character based output, but rather write to the screen directly. Note that scrolling may reduce the output speed significantly and is not included in the BIOS throughput test by COMPTEST. o Speed of video output via DOS: The speed is given in characters output per second as measured for the DOS functions #9 (print string). Note that the DOS file functions can also be used to print to the screen and that the output speed for these function may differ from the output speed reported. Also, the output speed may depend on the string length of the string to be printed due to varying amount of overhead while calling DOS. Since an ANSI driver usually causes a much slower DOS video output due to the need of the driver to check the output stream for interpretable sequences, COMPTEST states if the speed shown refers to the output speed with or without an ANSI driver present. To check for an ANSI driver, COMPTEST prints an ANSI ESC sequence that causes an ANSI driver to report the cursor position by inserting the result string into the keyboard buffer. COMPTEST then checks if this information has arrived in the keyboard buffer and assumes the presence of an ANSI device driver if it finds the information in the keyboard buffer. Besides the use of an ANSI or similar terminal driver (e.g. EANSI, NANSI), the use of a 386 memory manager such a 386MAX, QEMM, or EMM386 can slow down DOS video throughput as the use of these programs causes a higher overhead in the interrupt calls used in the code that prints to the screen. o DOS version: Shows the DOS version as reported by a call to the DOSGetVersion function of MS-DOS. For version 5.0 or later, this may not be the true DOS version, since the DOS version number reported to an application can be manipulated using the SETVER utility of DOS. o Standard benchmarks: COMPTEST uses three widely known standard benchmarks to provide some measurements of system performance. Since results for these benchmarks depend not only on the speed of the hardware, but also on the code quality of the compiler, only the relative performance to the original IBM PC displayed by COMPTEST is really significant. The reference numbers used in COMPTEST were determined using my own fast replacement for Turbo Pascal 6.0's run-time library (available as TPL60N19.ZIP from garbo.uwasa.fi and additional ftp sites). The compiler switch settings were the same as those found in the source code of COMPTEST 2.59. If you use another compiler, e.g. Stony Brook Pascal+, which is an optimizing compiler mostly compatible with TP 6.0, or use other switch settings, you *must* determine new reference values for the IBM PC if the PC relative performance numbers are to be of any use. o Dhrystones: The results of running the Dhrystone benchmark, a synthetic benchmark that is supposedly representative of integer applications. Note that Dhrystone performance depends on the hardware as much as on the compiler. Therefore, Dhrystone numbers by other system test programs may be higher or lower as those reported by COMPTEST, depending on whether or not they were compiled with an optimizing compiler or run as a 16-bit or a 32-bit program. There are different versions of Dhrystone, the version used here (2.1) is the latest available from the author of the benchmark, Reinhold Weicker. The Dhrystone code fits well into a rather small cache (8 KB will be sufficient), so for systems with CPU caches it tests only CPU performance, *not* the performance of the memory system. To make the Dhrystone performance as determined by COMPTEST useful, the relative performance as compared to the original IBM-PC is given. o Whetstones: The results of running the Whetstone benchmark, a synthetic benchmark that stresses mainly floating point performance, including trans- cendental functions like Sin or Exp. As the Dhrystone numbers, the results of the Whetstone benchmark depend as much on the hardware as on the code quality of the compiler (whether optimizing or not), although the compiler dependency is usually somewhat less than with the Dhrystone benchmark. Therefore, Whetstone numbers as determined by COMPTEST should not be compared to those determined by other programs. To make Whetstone results useful, the performance is also rated in comparison with the original IBM-PC. Note that the test uses software emulation of the coprocessor if the machine tested does not have an 80x87 mathematical coprocessor, and that in this case the performance is compared to the equivalent PC configuration, that is a PC without an 8087. There are two versions of the Whetstone benchmark, an older version derived from the original article published in 1976 and a newer version that includes sanity checks. The latest available version acquired from one of the original authors (Brian Wichmann) is used here. o MFLOPS: This benchmark result tells you how many millions of basic floating point operations (add, subtract, multiply) the tested machine is able to execute per second. This number is determined by running an older version of the Lawrence Livermoore Loops, a set of 14 kernels taken from *real* number crunching programs and computing the average MFLOPS (Millions of FLoating-point OPerations per Second). There is a newer suite of LLL out that uses 24 kernels and provides more detailed diagnostics of the floating point performance. Due to its size, it could not easily be integrated into COMPTEST, so the older (and simpler) version was used. The LLL benchmark uses about 60 KB of RAM, so results may be influenced by the size of the CPU cache, if any is installed. For reference, the MFLOPS are compared to the performance of an IBM-PC with 8087 (if the tested machine also has a coprocessor) or to a plain IBM-PC using the software emulator (if the machine tested does *not* have a coprocessor). As with the other benchmarks, the LLL performance depends not only on the hardware, but also on the compiler used. Highly optimizing compilers that make use of 32-bit instructions where possible would give an MFLOPS rating that is about 50% higher. o Hard disk data: COMPTEST usually is able to test all hard disks in your system, regard- less of whether they use the ST506, IDE, ESDI, or SCSI interface. How- ever, it may fail to detect some special types of hard disks like the removable data packs found on some Tandon computers that use special drivers to hook into the DOS file system. o Hard disk geometry (# cylinders, # read/write heads, sectors per track): These disk parameters are given as reported by the BIOS. The parameters given may not reflect the physical geometry of the disk. For example, if a disks uses the zone bit recording technique, there is no fixed ratio of sectors per track, rather the number of sectors per track is greater in the outer zones and smaller in the inner zones. So the parameters given reflect the logical layout of the disk as seen by the BIOS, which may or may not coincide with the physical layout of the disk. o Hard disk storage capacity: The *formatted* capacity of the disk is given in bytes and MB. Note that a MB contains 1024x1024 = 1,048,576 bytes if computed correctly. Some disk manufacturers (e.g. Quantum) state the storage capacity in MBs consisting of only 1,000,000 bytes. Therefore, the capacity in MB as reported by COMPTEST may be lower than the capacity the manufacturer claims for the disk. Also, the capacity you can use using DOS may be even smaller, since DOS allocates some disk memory to build allocation structures like partition tables or FATs. o Track-to-track seek time: The time it takes to move the read/write heads of your hard disks from one cylinder to an adjacent cylinder. The time reported may be zero when using certain disk cache programs (e.g. HyperDisk), as these suppress unnecessary head movements if no data is read or written in the process (as happens when COMPTEST does this test). Also, COMPTEST may be fooled by the so called translation modes mainly used by certain hard disks using the IDE interface to overcome limitations to the maximum number of cylinders set by the BIOS or to accommodate the fixed sectors/track scheme of PCs to the modern zone bit recording technique. With translation mode enabled, two logical tracks can reside on the same physical track, essentially nullifying the time to move between the logical tracks. COMPTEST moves the read/write heads over all tracks in single track steps and divides the total time by the number of tracks moved. This provides an average time, since movements between adjacent tracks may take different times depending on the absolute location of the tracks. o Average seek time: The time needed on the *average* to position the read/write heads over an arbitrarily selected cylinder on the disk. This time is roughly equal to the time it takes the heads to travel over one third the total numbers of tracks. It can be shown that, if tracks are selected at random from a uniform distribution on [0..MaxTrack] the average difference between any pair of track numbers is equal to one third the total numbers of tracks. Note however, that the assumption of a uniform distribution in track access patterns usually does *not* hold for practical file systems. Also, the average number of tracks traveled by read/write heads varies for the different zones of a disk using zone bit recording or using read/write queuing. For most disks however, the number reported by COMPTEST should be close to the number stated by the disk's manufacturer. When using certain disk cache programs, you will see very small times due to the fact that these programs suppress all unnecessary head movements. Note also that the average access time by definition is not equal to the tine needed on the average to access a specific sector on the disk. For this you have to add at least the rotational latency (the time needed until the read/write head reaches the designated sector on the track after having moved to the correct cylinder). This takes half of the time required for a full revolution of the disk in the average case, that is 8 ms for a disk spinning at 3600 rpm. Newer disks often spin at a higher speed of 4400 rpm or more to reduce rotational latency. o maximum throughput: COMPTEST determines the maximum throughput of a disk similar to the well known CORETEST disk test program by repeatedly reading the same block from disk. It uses the low level functions provided by the BIOS to maximize performance. The amount of data read is the data on one cylinder or 63 KBytes, whichever is smaller. Therefore, no movement of the read/write heads occurs during the read test. The disk's read throughput is determined for the first and the last cylinder on the disk. For hard disks using the zone bit recording technique, the throughput on the outermost cylinder can be about 50% higher than on the innermost cylinder, since more data is recorded on the outer cylinders. Since COMPTEST reports the maximum throughput, it always reports the higher of the two transfer rates it determines. The transfer rate in real applications is usually much lower than the maximum transfer rate reported by COMPTEST. By repeatedly reading the same block from disk, COMPTEST causes the block to be read into the track buffers and on-disk hardware caches present on most modern disks or the cache memory of a caching controller. If the block fits completely into such a cache, the transfer rate measured is actually the transfer rate between the buffer/cache and the system memory. A more realistic transfer rate could be determined by completely reading the data on several adjacent tracks. Since every data item is read only once, a cache will not inflate the transfer rate. The transfer rate determined by this process is called the linear read rate. It is used to measure disk performance in programs such as PCTOOLS 7.1's SI. The linear read rate can be useful to determine disk performance for operations on large files that are contiguous and are read sequentially with only a small amount of head movement. Many applications use random access files, though, and files may not be stored in contiguous form on the disk. In these circumstances, there is a considerable amount of head movement and the seek time and rotational latency cause overhead that further reduces the effective transfer rate available to applications. Note that COMPTEST uses the BIOS to access the disk, while applications make use of an operating system's file system that introduces additional overhead. Also, write access to a disk may be significantly slower than read accesses. Hardware caches on the disk or on the disk controller and software caches like HYPERDISK and SMARTDRV can provide better disk performance by storing frequently used data in cache memory that can be accessed faster than the disk itself. COMPTEST tries to determine the presence of a disk cache by repeatedly reading a small amount of data from different (non-adjacent) tracks. If no cache is present, the head movement to access the different tracks is the reason that the read time can not fall below a certain level due to the physical limits that prevent a reduction in average seek times below about 12 ms. If a cache is present, all data can be hold in the cache memory after the first read access and no additional head movements take place, thereby causing fast execution of COMPTEST's test to determine presence of a disk cache. COMPTEST does not differentiate between hardware and software caches.