I am new to NASM and new to 8086 assembly too (but I am familiar with the instruction set). I am no noob to assembly, having worked on 10+ CPUs over the years.
I have recently built some hardware, a
8088 "PC"-lite clone. I have been able to run the Tandy1000 BIOS and confirm it got through the BIOS start-up. It gets stuck at the floppy access code, which is not surprise, as I haven't yet built the floppy disk controller PCB! I found on Github a disassembly, which used IDA Pro, for the Tandy1000 BIOS. This has been helpful in getting my hardware to work.
I decided to use NASM since it is modern and well supported.
I would like to re-code the BIOS, just taking the parts I am interested in, and changing parts as necessary. I can use the disassembled code and then "knock" it into shape for NASM.
I have been using the XT IDe Universal BIOS (which uses NASM) as a reference, but it is very complicated.
Although I am new to 8086 assembly, perhaps this is no drawback, because the NASM syntax is different to other assemblers I am aware of (TASM, MASM, Watcom, etc).
My first question concerns accessing data at a fixed location. I want to access the BIOS variables at 0x400 and a temporary data structure at 0xbbee0 (memory in the GFX card).
The disassembled code looks like this example ...
F000:E05B mov ax, 40h ; '@'
F000:E05E mov ds, ax ; ds = 0x40
F000:E060 mov ax, 0BBE8h
F000:E063 mov es, ax ; es = 0xbbe8
F000:E065 xor ax, ax
F000:E067 cmp word ptr ds:p400.POST_reset_flag, 1234h ; soft reset?
F000:E06D jnz short l0 ; no, jump
F000:E06F mov ax, ds:p400.keyb_status_flags_1
F000:E072 and ax, 6060h ; filter out insert / caps
F000:E075
F000:E075 l0:
F000:E075 mov es:pbbe80.shadow_keyb_status_flags_1, ax
F000:E079 mov ax, ds:p400.POST_reset_flag
F000:E07C mov es:pbbe80.shadow_POST_reset_flag, ax
Now I have taken the BIOS variables struct from the XT IDE Universal BIOS, and I also see how these are accessed. What is not clear is how to code the segment part. Is there a recommended way? Can I do a "%define BIOS 0x400" and then for the "mov ax,40h" (above) with some more readable code e.g. "mov ax SEG BIOS(
![Huh ???](http://forum.nasm.us/Smileys/default/huh.gif)
)". I think I know how to code the reads of these locations e.g. "mov ax, [ds:BIOS.POST_reset_flag]. I am happy that the offset part gets calculated from the struct. I could leave the hard coded segment values in, but this looks tacky to me.
BTW I have read the PDF documents, but of course I am on a steep learning curve. I tried searching for NASM examples, but many are for apps for modern processors, not for embedded ROM code.