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Z-System Corne≥ (c) by Jay Sage The Computer Journal, Issue 39 Reproduced with permission of author and publisher For this issue I will discuss some unique new capabilities made possibleì by NZCOM that have already proved their value and that I hope will beì exploited to a much greater extent in the future. I originally had severalì other issues on my agenda, but Lee A. Hart sent me such an interestingì article that I wanted to leave plenty of room for it. System Enhancements Using NZCOM I'm afraid that many people think of NZCOM as just a way for unskilledì users to get Z-System running on their computers. It's true that NZCOMì accomplishes that, but, as I have asserted often before, NZCOM does muchì more than that. It offers possibilities that a manually installed Z-Systemì cannot achieve. I would like to describe one of them here and will do soì first in the context of a problem that arose with the new ZSDOS and ZDDOSì disk operating systems on some computers. Because CP/M was created originally for the 8080 microprocessor, a numberì of BIOS implementors (the BIOS, or Basic Input/Output System, is theì hardware-dependent part of CP/M) felt that they could make free use of theì additional registers introduced with the Z80 chip. These registers includedì two index registers, called IX and IY, and a duplicate set of the standardì registers denoted with primes on the names (e.g., B' or HL'). This was perhaps excusable at the time, but it is poor programmingì practice for an operating system to change anything other than what isì explicitly indicated in the specifications. Most BIOS writers who have usedì the Zilog registers have been careful to restore the original values beforeì exit from the BIOS routines. Unfortunately, a few BIOSes fail to do that. ì Among them are the following: Epson QX10, Zorba, Televideo 803 and TPC-1,ì Oneac On, and the Osborne Executive. The Bondwell is on the suspect list,ì and there are probably others we don't know about yet. The (mis)use of these registers poses no problem for programs written toì run on the 8080, but today we are rapidly moving beyond the limitations ofì the 8080 and making extensive use of the Z80 registers to pack more powerì into operating system components and application programs. Today, the Z¡ System, true to its name, is intended to run only on the Z80 or upwardlyì compatible processors like the HD64180, Z180, or Z280. Several users who purchased ZDOS (that is ZSDOS and ZDDOS) found that itì would not work properly on their computers. An investigation turned up theì fact that the BIOSes in those computers were modifying the index registers. ì This also explained why those same users had been experiencing strangeì problems with JetLDR, Bridger Mitchell's superb Z-System module loader. Itìèalso explained some mysterious problems I was having with a number ofì programs (for example, EDITNDR) on my Televideo 803! The question was whatì to do about the problem. In the ancient days, when computers always came with the source to theirì BIOS and their owners were always intimately familiar with the proceduresì for rebuilding their operating systems, the solution would have been toì rewrite the BIOS with the proper PUSH IX and POP IX instructions to preserveì the index register values. But what could we do today for nonprogrammersì and those without BIOS source code? NZCOM provided the answer quite nicely! As I explained in a column long ago, NZCOM works by creating what I callì a virtual BIOS (I'll call it VBIOS) lower in memory in order to open upì space for the Z-System modules between it and the real BIOS (often calledì the custom BIOS or CBIOS). The source for this virtual BIOS is available. ì Except for the warmboot code and some minor complications for IOPì (input/output processor) support, the standard NZCOM VBIOS module justì vectors calls that come to it up to the real BIOS. But no one says this is all it is allowed to do. ZDOS authors Camì Cotrill and Hal Bower found it quite easy to surround the vectors with codeì to save and restore registers. First they released ZSNZBI11.LBR, whichì contained the source and ZRL file (loadable by NZCOM) for a VBIOS thatì preserved the IX and IY registers for all disk function calls. Later theyì discovered that some of the machines changed the index registers even forì console I/O function calls, and others changed the alternate registers. ì They then wrote ZSNZBI12.LBR, whose VBIOS preserves all the registers forì all BIOS functions. Instead of having the virtual BIOS routines jump directly to the realì BIOS, they jump to an intermediate entry point. For example, the list¡ status vector in the jump table has a JP ILSTST (intermediate list status),ì and the code at ILSTST is ILSTST: LD A,45 JR DOBIOS The offset for the BIOS function is placed in the A register and thenì control is transferred to a general BIOS-calling routine shown in Table 1ì that implements the register protection. The routine JPHL referenced thereì contains only the code line JP (HL), which vectors the CPU off to the BIOSì with a return to the DOBIOS code. ----------------------------------------------------------------------------- DOBIOS: LD HL,CBIOS ; Start with address of real BIOS ADD A,L ; Add offset in A (never a LD L,A ; ..carry since on page boundary) EXX ; Swap to alternate registers LD (HLP),HL ; Save them all in memory LD (DEP),DE LD (BCP),BCè LD (IXREG),IX ; Save index registers, too LD (IYREG),IY EXX ; Back to regular registers EX AF,AF' ; Swap to alternate PSW PUSH AF ; Save it on stack EX AF,AF' ; Back to original PSW CALL JPHL ; Actually call the BIOS! EXX ; Restore alternate and index LD HL,(HLP) ; ..registers LD DE,(DEP) LD BC,(BCP) LD IX,(IXREG) EX AF,AF' ; Alternate PSW, too POP AF EX AF,AF' RET ; Register save area BCP: DEFS 2 ; BC' DEP: DEFS 2 ; DE' HLP: DEFS 2 ; HL' IXREG: DEFS 2 ; IX IYREG: DEFS 2 ; IY END Table 1. Code from ZSNZBI12.Z80 that preserves all index and alternateì registers across BIOS calls in NZCOM. ----------------------------------------------------------------------------- To use this replacement VBIOS, you have to run MKZCM and create an NZCOMì system with 4 records allocated for the BIOS instead of the standard 2. ì Because the BIOS must start on a page rather than just a record boundary,ì MKZCM will sometimes make automatic adjustments to the BIOS size. ì Therefore, you should specify changes in MKZCM starting with the higher¡ numbered modules; the adjustment in the BIOS allocation should be made last. ì If an attempt to enter a value of 4 results in MKZCM using 5, then you couldì go back (if you don't like wasting memory) and make one of the other modulesì (such as the NDR) one record larger and then respecify a 4-record BIOS. There are many other ways that NZCOM can be used to introduce systemì enhancements without having to make changes in the real BIOS. As anì example, we will show how to add support for the drive vector in environmentì descriptors of type 80H and above (implemented with NZCOM and Z3PLUS). Theì drive vector is a 16-bit value stored as a word beginning at offset 34H inì the environment descriptor. It specifies which disk drives are actuallyì implemented on a system. The lowest order bit in the word is for drive Aì and the highest for drive P. A zero in a bit position indicates that theì corresponding drive is not available. For example, on my SB180 the bytes atì addresses ENV+34H and ENV+35H were 7FH and 00H, respectively. Thus the wordìèvalue is 007FH or 0000,0000,0111,1111 binary, indicating that I had drivesì A, B, C, D, E, F, and G. When the hard disk E partition developed a problemì that made it unusable, I changed the 7FH value to 6FH, thereby disablingì drive E. You can display the drive vector using menu selection 3 in theì SHOW program (ZSHOW for Z3PLUS). The ZCPR34 command processor knows about the drive vector and will notì allow command references to unsupported drives. But what about programsì that you run? Unfortunately, the BIOS generally knows only which drives areì potentially implemented, and it may try to access a nonexistent drive. Whenì 'smart' BIOSes encounter this problem, they often prompt the user as to whatì to do next. This is fine if you are sitting at the console and can takeì appropriate action to recover, but if the system is being run remotely,ì there is generally no way for the remote user to recover. In fact, becauseì the 'smart' BIOS uses direct hardware calls to display the "Abort, Retry,ì Ignore?" message rather than calls through the BIOS vector table, the remoteì user does not even see the message and just thinks the system has crashed. ì My Z-Node got hung once that way when I forgot to put a diskette back into aì floppy drive. A caller attempted to access it, and when I got home, theì BIOS was dutifully beeping at me and waiting for me to tell it what to do. My first stab at writing a VBIOS that observes the drive vectorì restrictions is shown in Table 2. Now that I have read Lee Hart's column, Iì am sure that this code can be made shorter, faster, or both! But I willì leave that as an exercise for the reader. (I already see one place where Iì could save a byte.) The listing assumes you are starting with the ZSNZBIO described earlier. ì Just add the extra equate for DRVEC under the /_ENV_/ common block andì replace the simple ISELDK (intermediate select disk) code with the slightlyì more complex version shown in the Table. I put this on my Televideo, andì the results were most pleasant. Now when I attempt to access a nonexistentì drive, the system does not force a direct-CBIOS warmboot that drops me outì of NZCOM. ----------------------------------------------------------------------------- ; Add DRVEC definition in the ENV common COMMON /_ENV_/ Z3ENV: DRVEC EQU Z3ENV+34H ; Drive vector CCP EQU Z3ENV+3FH DOS EQU Z3ENV+42H ; Modify ISELDK as follows and place it after the DOBIOS code and before ; the data area for register storage. ISELDK: LD HL,(DRVEC) ; Get drive vector LD A,16 ; Subtract requested drive SUB C ; .. from 16è LD B,A ; .. and put into B ISELDK1: ADD HL,HL ; Move bits left into carry DJNZ ISELDK1 ; Loop 16-<drive> times LD HL,0 ; BIOS return code for invalid drive RET NC ; Return if drive vector bit not set LD A,27 ; Otherwise, use CBIOS function JR DOBIOS ; .. at offset 27 Table 2. Code added to virtual BIOS to support the environment drive vectorì at the BIOS level. ----------------------------------------------------------------------------- These two examples of system enhancements by no means exhaust theì possibilities. One can implement all kinds of additional features andì drivers right in the NZCOM VBIOS. Joe Wright suggested early during NZCOMì development that one should create an absolutely stripped down CBIOS, oneì that contains only the functions that are absolutely necessary to get theì system running and then implement all the bells and whistles in the VBIOS. ì These extra features would include things like RAM-disk drivers, keyboardì type-ahead buffers, logical drive swapping facilities, and disk errorì recovery management routines. With this strategy, one can actually achieveì a larger TPA with an NZCOM system than one had under the standard CP/Mì system, since the CBIOS can be made smaller and the fancy features droppedì when a larger TPA is more important. For example, the "Abort, Retry, Ignore" message should be implemented inì the VBIOS, with the CBIOS returning from disk errors with standard errorì codes. With the normal VBIOS, the error will simply be passed back to theì DOS, which will report the error in its usual way ("BDOS ERROR on..." in theì case of the Digital Research BDOS). A more elaborate VBIOS can detect theì error, report it to the user, and allow the operation to be retried. Whenì the system is running in remote mode, either the simpler VBIOS can be usedì or the prompt can be vectored properly through the jump table so that theì remote user will be able to deal with the problem. Similarly, one should be able to handle the swapping of logical driveì names in the VBIOS. There are a couple of pitfalls to watch out for,ì however. If you change logical names, you better make sure that the diskì system is reset, probably both before and after the swap. You also betterì make sure that NZCOM can still find its CCP file, which is normally kept inì directory A15:. If you swap the A drive without providing a copy of thisì CCP in the new A drive, you'll be in serious trouble. Of course, theì swapping would be handled by a utility program, and it would worry aboutì these requirements. The VBIOS would simply have the code for translatingì references to a logical drive value in register C into a possibly differentì physical drive value. I hope that this short discussion has given some of you ideas forì imaginative applications of the new capability offered by the NZCOM virtualì BIOS. If so, I would love to hear about them and to see sample code.è [This article was originally published in issue 39 of The Computer Journal, P.O. Box 12, South Plainfield, NJ 07080-0012 and is reproduced with the permission of the author and the publisher. Further reproduction for non- commercial purposes is authorized. This copyright notice must be retained. (c) Copyright 1989, 1991 Socrates Press and respective authors]