On 11/16/2024 10:04 AM, Royce Taft wrote:
Any recommendations on where to post updates? Should I just start a new thread
here? Or start a thread in the Slabtop Computers Discord server?
Discord is a good place to ask questions and discuss items, but I think
updates are best done on a discussion forum or here, this thread is
fine. Lists like this and forums are indexed, and so your updates will
reach a larger audience and survive time better.
Here’s a quick update:
The boot ROM disassembled nicely in Ghidra. It was handy labeling each part of
the memory and I/O maps with their function, and using the FDC and CRTC
controller’s datasheets to resolve exactly what is done when certain registers
are read or written with specific data. I’m still working off a lot of
assumptions, and many of the labels I’ve appended are educated guesses at best.
IMpressive
The ROM routine initializes some hardware and zeros out and sets up some data
in RAM2, then it copies a decent section of data to RAM2. This data is actually
program code, and once it’s copied to RAM, the program jumps to run it. One of
the first things the program in RAM2 does is hit the bank select register so
ROM is no longer in the picture, and RAM1 is swapped into its place in the
memory map. From here on out, the system is running code from RAM. Easy for us
to modify with our own code running from a custom boot disk!
Nice!
I identified the routine which reads 256 bytes of data contained in sector 1 of
track 0. That sector contains the IPL for the system. According to the
spreadsheet in Steve Adolph’s files, the first 128 bytes of that sector contain
disk basic IPL, and the second 128 bytes contains DOS IPL. I’m in total
agreement with that assessment. Once sector 1 is read and stored into RAM, the
system checks to see that this is indeed a valid boot sector by comparing the
last byte of the data read from the sector to 0x4b. Prior to reading the disk,
that location in memory is either 0 or uninitialized. Once a proper boot disk
sector 1 is read, that location in memory will contain 0x4b. Once this is true,
the system jumps to run the code which came from the second 128 bytes of the
boot disk sector 1, which is the DOS IPL. I copied the sector 1 bytes from the
DAT files extracted from the disk image by Steve Adolph (disk basic IPL + DOS
IPL) into memory in Ghidra at the correct location, and it disassembled the DOS
IPL! After that, I needed a little break. I’ll get back to it soon.
That's a lot!
I identified routines which display text on the screen prompting the user to
insert a disk, and also found routines to display the various error conditions
such as a non system disk inserted, drive didn’t seek to track 0, no index
pulse, etc. Returning from a CALL with the carry flag set appears to indicate
a failure of the process for drive control or disk reading.
There are a number of various pointers in RAM which are modified to contain
different addresses based on certain conditions, and there are even small
portions of self modifying code, actually changing an upcoming instruction
based on certain conditions. The engineers took full advantage of the code
running from RAM. It’s a lot for me to follow.
I’m not sure I’m experienced enough to be “the” person attempting to
disassemble and make modifications to the code, but I’m certainly capable of
learning.
I'm also new at this environment, but lots of folks here are more
knowledgeable, and we can all assist. I can help with designing a PCB
and such.
Is there a good place for me to share the Ghidra files for others to poke
around to learn or assist? I’m a noobie with git (and Z80 code), but I’m not
so embarrassed by my inexperience to shy away from sharing my work.
A long time ago, I decided that I was not going to be bothered by my
inexperience in sharing stuff like this online. I think GitHub (or
GitLab or BitBucket, etc.) are good to use, and allow many folks to
collaborate on these. I use GitHub, and then use git on my PC (I run
Windows, but Linux/Mac work fine as well), and it is pretty easy to make
changes locally and "push" those to the archive online. I think that's
best.
I am happy to host the repo on github.com/go4retro/<projectname> and add
you as a collaborator (and I'd be happy to get on discord chat/video and
walk you through how to easily sync local stuff and the online repo so
it's not a source of stress).
Jim
--
RETRO Innovations, Contemporary Gear for Classic Systems
www.go4retro.com
store.go4retro.com
