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Text File | 1993-05-02 | 23KB | 557 lines

Subject..: Low level Routines in Microsoft BASIC Author...: George Spafford Version..: 1.1 Date.....: May 2, 1993 This small tutorial package is aimed at users who have a working knowledge of the Microsoft BASIC language. It is not aimed at the novice programmer. I hope that you find the information in this tutorial useful. If you do, please register this shareware. As a programmer, you know that this type of an endeavor requires a substantial investment of time and effort. If I am to continue it, I need your support. Section Outline: 1. Getting Started a. Binary Numbers b. Hexidecimal Numbers c. Decimal Numbers d. Registers e. Bit manipulations 2. Interrupts 3. Peeks 4. Pokes 5. Summary 6. Revision History 7. Please register this shareware! ----------------------- NOTES ABOUT SAMPLE CODE: ----------------------- Look at the sample .BAS files that are included with this package. Print them out and refer to them while you read this text. The .BAS files are heavily commented and should help to clarify some of your questions. If you are anything like me, looking at code clarifies my understanding of a problem immensely. If you intend to use any of the sample programs after the 30 day evaluation period, you must register the entire tutorial package. ----------------------- 1. GETTING STARTED ----------------------- I like BASIC! While many people make excuses for using it, I find it a pleasant language to develop in. By adding libraries or writing my own add-ins, I can perform just about any task I need to. As far as code reusability goes, if I used modularized programming with functions and subroutines, it is quite high. Note, in order for a person to see most of the code that I wrote for the demonstration programs, I refrained from functions and subroutines in order to adopt a top-down approach. The exception to this is MAKEWORD& - which is a function that I used in several demos (I called it INT2LONG& in WORD.BAS). Alright, let's get started. A. Binary Numbers The binary number system reflects that a computer circuit can only be "on" or "off" - nothing else. Thus, a number scheme is used to represent any number by means of a "1" or a "0". Any number can be computed by using the equation: 2^n + ... 2^3 + 2^2 + 2^1 + 2^0 Numbers, bit sequences are evaluated from right to left. Let's take a look at some examples of this: 8 Bits Decimal -------- = ------- 00000000 0 00000001 1 00000010 2 00000011 3 00000100 4 ------ 10000000 128 8 Bit Positions 00000000 |||||||| |||||||| Exponent |||||||| -------- |||||||\----- 0 ||||||\------ 1 |||||\------- 2 ||||\-------- 3 |||\--------- 4 ||\---------- 5 |\----------- 6 \------------ 7 Long Hand Examples: 01001101 = 0^7 + 2^6 + 0^5 + 0^4 + 2^3 + 2^2 + 0^1 + 2^0 = 0 + 64 + 0 + 0 + 8 + 4 + 0 + 1 = 77 10011100 = 2^7 + 0^6 + 0^5 + 2^4 + 2^3 + 2^2 + 0^1 + 0^0 = 128 + 0 + 0 + 16 + 8 + 4 + 0 + 0 = 156 00000111 = 0^7 + 0^6 + 0^5 + 0^4 + 0^3 + 2^2 + 2^1 + 2^0 = 0 + 0 + 0 + 0 + 0 + 4 + 2 + 1 = 7 Note: If a bit position is 0 then the value is 0 - why? The actual notation for bit calculation can be identified as (1 x 2^n) if a bit is on or (0 x 2^n) if a bit is off. I just skipped that particular step in my examples. 16 Bit Positions: 00000000 00000000 Exponent |||||||| |||||||| -------- |||||||| |||||||\------ 0 |||||||| ||||||\------- 1 |||||||| |||||\-------- 2 |||||||| ||||\--------- 3 |||||||| |||\---------- 4 |||||||| ||\----------- 5 |||||||| |\------------ 6 |||||||| \------------- 7 |||||||| |||||||\--------------- 8 ||||||\---------------- 9 |||||\----------------- 10 ||||\------------------ 11 |||\------------------- 12 ||\-------------------- 13 |\--------------------- 14 \---------------------- 15 A Long Hand Example: 01000100 10010011 = 0^15 + 2^14 + 0^13 + 0^12 + 0^11 + 2^10 + 0^9 + 0^8 + 2^7 + 0^6 + +0^5 + 2^4 + 0^3 + 0^2 + 2^1 + 2^0 = 0 + 16384+ 0 + 0 + 0 + 1024 + 0 + 0 + 128 + 0 + +0 + 16 + 0 + 0 + 2 + 1 = 17,555 B. HEXADECIMAL NUMBERS The hexadecimal numbering system is a hold over from the days when there were 4-bit processors. If you have every studied statistics, then you know if there are four available bit positions, there are 16 possible bit combinations (2^4). Thus, a base 16 number system was developed. Decimal Hex Binary ------- --- ------- 0 = 0 = 0000 1 1 0001 2 2 0010 3 3 0011 4 4 0100 5 5 0101 6 6 0110 7 7 0111 8 8 1000 9 9 1001 10 A 1010 11 B 1011 12 C 1100 13 D 1101 14 E 1110 15 F 1111 Since Hex is base 16, start from the right of a value and work your way to the left - just as you did with binary. 03F8 = (0 * 16^3) + (3 * 16^2) + (F * 16^1) + (8 * 16^0) = 0 + 3 * 256 + 15* 16 + 8 * 1 = 0 + 768 + 240 + 8 = 1,016 2F = (2 * 16^1) + (F * 16^0) = 32 + 15 = 47 BASIC can process directly coded hex by prefixing the hex value with "&H". &H03F8 <- to BASIC, this means 3F8 Hex, 1,016 decimal C. DECIMAL NUMBERS The decimal number system is what most of us are familiar with so I will not go into great details about it. BASIC, however, does discriminate between declarations of numeric variables: Variable Memory Suffix Meaning Range Requirements -------- ---------- ------------------------------ ------------ % integer -32,768 to 32,767 (2 bytes) & long -2,147,483,648 to 2,147,483,647 (4 bytes) ! single precision (4 bytes) # double precision (8 bytes) D. REGISTERS Simply put, registers are memory locations within the processor. 16 bit 8 bit Usage -- -- ----------------- AX AL Accumulator AH BX BL Base BH CX CL Count CH DX DL Data DH DI Destination index SI Source index SP Stack pointer BP Base pointer DS Data segment CS Code segment ES Extra segment SS Stack Segment AX, BX, CX and DX are 16-bit registers that can be further broken down into two 8-bit registers each. The Low register contains bits 0 to 7 and the High register contains bits 8 to 15. Flag Registers: Bit Position Definition ---------- ------------------------------- 0 Carry Flag 2 Parity Flag 4 Auxiliary Flag 6 Zero Flag 7 Sign Flag 8 Trap Flag 9 Interrupt Flag A Direction Flag B Overflow Flag Note: You will see the Carry Flag frequently used to indicate whether an error has taken place or not. E. BIT MANIPULATIONS Many of the functions at the machine level require that you operate at the bit level rather than at the byte level. While BASIC does not offer a variable at the bit level, bits can be manipulated through the use of the logical boolean operators. Remember when we discussed the bits earlier? That 00000001 = 1 and so forth? Consider the following, what is the fastest way to determine if a number is odd or even? Give up? Take a look at this code fragment: INPUT "Enter a number ",number IF number AND 1 THEN PRINT "That is an odd number" ELSE PRINT "That is an even number" END IF The key in this fragment is the usage of the AND operator. Take a look at these following odd numbers: 00000001 = 1 00000011 3 00000101 5 00000111 7 What do they have in common? They all have the right most bit (bit 0) turned on. The AND operator evaluates numbers and if the number to the right of the and has the same bit(s) on as the number to the left, then the answer is true. Bits Decimal -------- ------- 00000001 1 00000010 2 00000100 4 00001000 8 00010000 16 00100000 32 01000000 64 10000000 128 Examples: 96 AND 64 01100000 01000000 TRUE Bit 6 matches 56 AND 8 00111000 00001000 TRUE Bit 3 matches 56 AND 2 00111000 00000010 FALSE Bits do NOT match 64 AND 8 01000000 00001000 FALSE Bits do NOT match Why does this come in handy? Let's say you want to test the FLAGS register after doing an INTERRUPT call in BASIC. You want to know if it has been turned on due to an error: IF OutRegs.Flags AND 1 THEN PRINT "Oh no, an error fried your hard drive!" END IF Let's say you use Crescent Software's excellent QuickPak Pro library for DOS (one of my favorites). Many of its file information subroutines return a byte field that contains the file's attributes. How do you test the attributes? DOS uses a byte to hold the information. Its 8 bits are used as follows: Bit Use --- --- 0 Read Only 1 Hidden File 2 System File 3 Volume Name 4 Directory 5 Archive Bit By now, you should know how to test for these, but let's cover it once more: IF attrib AND 1 THEN PRINT "Read Only File" IF attrib AND 2 THEN PRINT "Hidden File IF attrib AND 4 THEN PRINT "System File" IF attrib AND 8 THEN PRINT "Volume Name" IF attrib AND 16 THEN PRINT "Directory Name" IF attrib AND 32 THEN PRINT "Archive File" If you feel daring, you can get this information yourself by using Interrupt 21H, Function 43H, Subfunction 0. Input: AH=43H AL=00H DS=Segment DX=Offset DS:DX describes where to find the DOS file name. (File Name + Chr$(0). See CHGMOD.BAS for a closely related example.) OutPut: Carry Flag = 0 OK CX = File Attribute bit 1 = Read Only 2 = Hidden 3 = System 4 = Volume Name 5 = Archive Bit Carry Flag = 1 Error AX = 1 Unknown error 2 File not found 3 Path not found ------------------ 2. INTERRUPTS ------------------ The word for the day is "interrupt." An interrupt is a signal to the respective processor that its attention is required. The operation that is requesting the information returns a value that ranges from 0 to 255. This value is known as the "type code." The type code is then used to calculate a new address known as an "interrupt vector." This is the address of either a DOS or BIOS program that will deal with the call. Note, DOS stands for Disk Operating System and BIOS stands for Basic Input/Output System - just in case you didn't know. There are some minor differences that exist between DR DOS that is marketed by Novell and MS DOS that is by Microsoft (with version 5.00, I lump IBM DOS and MS DOS together in terms of functionality, but this has not always been the case). I bring this up, because you must be certain that your end-users can run your code. If you make a system call that only works in newer versions of DOS and not older ones, then you should check for the DOS version BEFORE your program executes that call. If you don't you will potentially have some upset customers with crashed systems. Also, if you require a TSR (Terminate Stay Resident) program to be in memory, such as LAN (Local Area Network) software, for an interrupt to work, check for it BEFORE you call it!!!! ------------ 3. PEEK ------------ PEEK is a command that resides in BASIC for the purpose of looking at a byte in memory. If you want to get the value of the first byte of the BIOS communications table, you would do the following: DEF SEG = &H0 A = PEEK(&H400) DEF SEG Line 1: This sets the current segment to &H0000 - the BIOS data area. (The start of the BIOS data area is actually 0000:0400). Line 2: Reads the integer value that exists at &H400, one byte only. &H400 is called an "offset." Line 3: This returns to the segment in which your BASIC programs is operating in. To PEEK word values, look at WORD.BAS. BASIC's internal PEEK function can only handle single bytes. WORD.BAS contains sample code to PEEK word values. ------------ 4. Poke ------------ POKE places a value into memory at a specified address. Whereas PEEK reads the memory, POKE writes to the memory. For example, let's say you want the PC to forget that COM1 exists: DEF SEG = &H0 POKE &H400,&H0 POKE &H401,&H0 DEF SEG Line 1: This sets the current segment to &H0000, the BIOS segment. Line 2: This writes the value &H0 to the memory location &H400. Remember, this location is called an offset. Line 3: This writes the value &H0 to the offset &H401. Line 4: Returns to the default BASIC segment. Voila! No more COM1 !! To POKE words, see the WORD.BAS tutor file. BASIC's POKE can only do one byte at a time that is annoying. WORD.BAS has sample code that you can use to poke words into memory at any location. POKEWORD could have done the above to the comm port in one call. --------------- 5. SUMMARY --------------- ::FURTHER READING:: There are two books that I recommend above all others: Tischer, Michael. PC INTERN, Abacus, Grand Rapids, MI, 1992. ISBN: 1-55755-145-6 Duncan, Ray. The MS-DOS Encyclopedia, Microsoft Press, Redmond, WA, 1988. ISBN: 1-55615-174-8 The encyclopedia is getting a little dated, but it is still useful. PC Intern is excellent! Tons and tons of information and well worth the $59.95. It has over 1200 pages and includes sample code on a disk. ------------------------ 6. Revision History ------------------------ v1.1 May 2, 1993 Added: CHGMOD.BAS Tutor on obtaining file directores, setting the DTA address, and setting file attributes. DUMPMEM.BAS Tutor on using PEEK. Runs from command line. (Functionally identical to READMEM.BAS) PORT.BAS Tutor on using the functions outlined in WORD.BAS to modify the I/O addresses in the BIOS data table. READMEM.BAS Tutor on using PEEK. Prompts user for answers. (Functionally identical to DUMPMEM.BAS) WORD.BAS Contains functions for converting integers to long integers and PEEKing words, and a subroutine to POKE words. Modified: Revised S3BASTUT.DOC to reflect the new programs and corrected as many of the typographical errors as I could find. v1.0 April 20, 1993 Initial release. ---------------- 7. Register ---------------- If you find this information useful, please consider registering it for $10 US. If there is interest, I will try to improve this tutorial. At the least, it served as an excellent review for me just to write it. If you use sample code from the tutorial, please have the courtesy to register the tutorial. If you want the latest copy of the tutor to be mailed to you with your registration, please add $5 to cover the diskette and mailing (the total would then be ($10 x # copies)+ $5). Please send your registration to: George Spafford 3003 Lakeshore Drive, #216 Saint Joseph, MI 49085 USA I can be reached on the following BBS systems: Michigan Online 19200-8-N-1 616-429-3414 [This board is local to me and I try to call it at least twice a week - usually more on weekends. EXEC-PC and Channel-1: These are pay BBS systems, so I am not going to list their phone numbers. ALL SAMPLE PROGRAMS ARE PART OF THE S3 SOFTWARE BASIC TUTORIAL. THEY ARE NOT TO BE CONSIDERED SEPARATE. IF YOU CHOOSE TO USE ANY OF THE PROGRAMS IN THIS PACKAGE AFTER A 30 DAY TRIAL PERIOD, YOU MUST REGISTER THE ENTIRE PACKAGE ON THE BASIS OF CONCURRENT USE. NOTE: The SDV file viewer is included with the tutorial for the use of viewing this .DOC file and other package files. If you register this tutorial, you may continue to use it on a single system concurrently. If you do not register this tutorial, but choose to continue using SDV, you must register it separately and details can be found in the SDV12.DOC file. "Concurrent Use" defined: I consider concurrent use to mean more than one PC running, or looking, at the tutorial and/or its programs at the same time. If you plan on having 5 PCs using the tutor, or any of the programs that comprise it, at the same time, then you must register 5 copies of the program. ============ LEGAL ============ THE S3 BASIC TUTORIAL AND ALL OF ITS ACCOMPANYING SAMPLE CODE AND PROGRAMS ARE DISTRIBUTED AS IS. THE AUTHOR (GEORGE SPAFFORD), OR ANY CONTRIBUTOR, MAKES NO WARRANTY OF ANY KIND, EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, WITH RESPECT TO THIS SOFTWARE AND DOCUMENTATION. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DAMAGES, INCLUDING LOST PROFITS, LOST SAVINGS, OR ANY OTHER INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OF OR THE INABILITY TO USE THIS PROGRAM. All products mentioned are the property of their respective companies. SDV and the S3 BASIC Tutorial are the copyright of George Spafford. All Rights Reserved.