CP/M/CPM_CDROM.iso/enterprs/cpm/utils/a/6502sim.lzh/ZXHINTS.DOC

Text File | 1991-01-16 | 8.8 KB | 161 lines

Hints & kinks for operation of ZX65: SIMULATING A MICRO

Written 7/80         R. M. Kruse


    There are a few things about ZX65 which are either not immedia-
tly evident in the original Dobb's article or which I have run across
since the article was submitted. These I will share with you in the hope
that they make your use of the package easier.

    MODIFICATIONS: If you compare the first few lines of both ZXLD
and ZX65 source code on this disk with the listings in Dobb's, you will
notice some differences. The reasons for these changes are twofold: 1)
since submission of the article, I have obtained a much better assembler
(namely the SD Systems Version 3.3). Use of this assembler required the
addition of the 'PSECT ABS' pseudo-op found in the new listing. Although
this assembler does not require the LABEL-$ convention in relative jump
instructions (as did V. 1.0) it tolerates them so they are still there.
2) I have cleaned up the mechanism for reassembling for different system
memory sizes through the use of the 'INCLUDE FILE' mechanism found in
both ZX65 and ZXLD. In case you're wondering, all instructions still
occupy the same absolute locations as in the Dobb's listing, with an
offset if your system is not 32K.

    RELOCATING THE SYSTEM: If for any reason you need to modify the
system to operate in a different memory configuration, follow these
steps:

    1. Using your editor, put a new value in the 'SIZE EQU XX'
       statement in the 'SIZE.ZX' file. This is normally the size
       (in decimal) of your CP/M system (i.e. 16K, 24K, 48K, etc)
       and upon assembly this puts the package from MEMTOP-1100H
       to MEMTOP-200H. You could, of course, locate the package
       just about anywhere by using oddball values in 'SIZE EQU
       XX' statement, but don't try this unless you know how to
       relocate your CBIOS. ZX65 expects CBIOS to be in the 512
       bytes of memory immediately following it.

    2. Assemble (and link, if necessary) both ZX65.ZSM and ZXLD
       .ZSM in order to create a .HEX file for each. (Note: if
       your assembler doesn't support the 'INCLUDE' mechanism,
       you will have to place the 'SIZE EQU XX' statement into
       both files manually using your editor. Also, you may have
       to do some minor syntax modifications to suit your ass-
       embler.)

    3. Don't try to use CP/M's LOAD utility. It can't handle the
       'load and boot' method used by ZX65. Instead, use DDT:

        a. First, load ZXLD by typing 'DDT ZXLD.HEX'. DDT
           will respond with 'PC 100  NEXT 10E' and a
           prompt.

        b. Next, load ZX65 by typing 'IZX65.HEX' <CR> and
           'Rdisp' <CR>. The hex value 'disp' is the offset
           necessary to fool DDT into loading a hex file
           at the 'wrong' address. If you have printed out
           the .PRN file for ZXLD, this value should be cal-
           culated for you at the 'OFFS EQU XXXX' statement
           (it should be a four-digit hex number ending in
           '10' such as 9210 for a 32K system.) You may
           also have DDT calculate it for you by typing
           'H110,ssss' where 'ssss' is MEMTOP-1100H for your
           system.

        c. You now have a memory image of ZX65 with a booter
           in the TPA. Exit DDT with a ^C (warm start) and
           type 'SAVE 15 ZX65.COM' to store it on disk as
           an executable command file.

        d. Invoke the package by typing 'ZX65'. Drive B
           should immediately operate briefly and a '>'
           prompt should appear on the screen. If not, go
           through the procedure again, and be sure that
           CBIOS is in its usual place.

    CP/M COMPATABILITY: As of this writing, I have not yet succum-
bed to Digital Research's efforts to get me to buy the new CP/M 2. From
what I have been able to figure out, it offers very little to me in the
way of advantages. Suffice it to say that ZX65 was written and debugged
using V 1.4, and I cannot say whether or not it will run under any other
version. (My guess is that it will work with V 1.3, but not v 2.X.) The
main reason for any incompatibility will probably have to do with the
fact that my self-contained mini DOS operates the disks and console dir-
ectly through CBIOS rather than by using calls to BDOS which is the more
conventional method. (There is a good reason for this madness which I
will explain shortly.) In any event, since you have spent your hard-
earned nickels for this copy, I suspect that you are resourceful enough
to overcome such problems and implement any necessary patches. If you
do come up with a running version patched for CP/M 2 or for some other
disk format (i.e. North Star etc.), how about submitting it for possible
publication in Dobb's? (No, I'm not on their payroll, but they are re-
ceptive to this type of material and lots of 'hard core' dedicated
computerists read their mag.)

    SOME NITTY-GRITTY: As promised, here is the explanation of why
I have interfaced through CBIOS rather than using BDOS: If you have
done any work at all with the 6502 processor, you will recall that page
zero of memory (0000H to 00FFH) is sacred to it. There are short forms
of many instructions for use on page zero, and the very powerful indirect
indexing modes always use page zero. Now ZX65 by design uses no memory
mapping at all...that is, if your 6502 program is ORG'd at 2000H, that
is exactly where you will load it. (This is in contrast to at least one
TRS80-based simulator that I know of.) Having gone this far, it is only
reasonable to expect that page zero will be totally available to the
6502 program. As you have probably realized by now, the catch here is
that BDOS also likes page zero, and uses large chunks of it during disk
accesses. With one possible limitation (which I will discuss in a bit)
ZX65 totally removes the host system from page zero...even the vectors
at the bottom of memory need not be saved.  ZX65 is, in fact, confined
to the top 1100H bytes of user memory, and ALL space below that is av-
ailable for 6502 programs and data. The one possible conflict that I
mentioned is system-dependent, and may or may not apply in your case.
CP/M V 1.4 has a reserved 'scratch' area of 16 bytes located from 0040H
to 004FH. Some disk controllers use this area for temporary data stor-
age during all disk transfers, whether initiated by CP/M or not. If this
is true of your controller, then you have very little choice but to ac-
cept the fact that this portion of page zero will be messed up whenever
you do a disk access from ZX65. There is still one alternative (I know
because I did it!): you may be able to move this scratch area out of
page zero into high memory. Even if you have this conflict, 6502 pro-
grams that use page zero only for temporary data storage will probably
run normally unless the 6502 program itself accesses the disk. (Yes, it
is possible, using the 'system' subroutine calls.)

    ZXDOS COMMENTS: Having destroyed BDOS as described above, I was
obliged to include an embedded disk operating system in ZX65. Now, ZXDOS
will never win any awards for versatility, but it is, I think, adequate
for the application. All internal disk access is confined to drive B (a
two drive system is assumed). ZXDOS files are not CP/M compatible. A dir-
ectory is maintained on track zero, and the remainder of the disk is
available for program/data storage. Each directory entry is 16 bytes long,
and contains the file name (format same as CP/M but file type has no spe-
cial meaning), the load address, the disk location, the number of sectors,
and an active/dead flag byte. During a directory display or search, the
directory is paged through a buffer within ZX65 one sector at a time,
continuing until an 'end of directory' mark is located. Files are stored
sequentially and on sequential sectors (no interleaving). A new file will
normally be appended at the tail end of existing files, unless the entire
file will fit into the space allocated for a dead file. Thus a rudimentary
form of dynamic disk space allocation is used, but a full disk will still
contain quite a few empty slots. Killing a file does not actually erase
anything, but simply resets the active status flag in the directory for
that entry. The 'initialize' command writes a properly formatted empty
directory to the disk. Although I have not done so, it should be possible
to use ZX65 with only one drive. The only change to the source code is at
line   37, where the instruction 'LD C,1' will have to be replaced with
'LD C,0'. To operate in this mode, first load ZX65, then immediately re-
move the system disk and insert the ZX65 data disk, and from this point
operation should be the same. This should work because once loaded, ZX65
does not itself require any further disk access.

    CONCLUSION: Have fun with ZX65!        -RMK



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