BIOS code looking for specific hard drives
Posted: Sat Sep 18, 2021 10:28 pm
All,
If you haven't read my previous post about "why I'm here", reading it will provide context to this post.
In this BIOS code snippet, the '286 processor is looking for any 1 of 5 Connor Peripherals hard drives starting at $76da. There are annotations, but I was hoping to gain some more insight for a better understanding of the code. I'm hoping to learn from any comments and or discussion.
Can anyone recommend a good disassembler?
Thanks,
-Shawn
If you haven't read my previous post about "why I'm here", reading it will provide context to this post.
In this BIOS code snippet, the '286 processor is looking for any 1 of 5 Connor Peripherals hard drives starting at $76da. There are annotations, but I was hoping to gain some more insight for a better understanding of the code. I'm hoping to learn from any comments and or discussion.
Can anyone recommend a good disassembler?
Thanks,
-Shawn
Code: Select all
0000769e 53 push bx {var_4_1}
0000769f 51 push cx {var_6_1}
000076a0 1e push ds {var_8_1}
000076a1 56 push si {var_a_1}
000076a2 06 push es {var_c_1}
000076a3 57 push di {var_e_1}
000076a4 ba0000 mov dx, 0x0
000076a7 e8f7a7 call sub_1ea1 {Call to retrive drive 40 character drive id string}
000076aa 7306 jae 0x76b2
000076ac ba55aa mov dx, 0xaa55
000076af eb7b jmp 0x772c
000076b2 b92800 mov cx, 0x28 {set counter for the id string}
000076b5 26803d2d cmp byte [es:di], 0x2d {find the "-" in the string}
000076b9 7407 je 0x76c2
000076bb 47 inc di
000076bc e2f7 loop 0x76b5
000076be f8 clc
000076bf eb68 jmp 0x7729
000076c2 47 inc di
000076c3 26803d20 cmp byte [es:di], 0x20 {move past the " " to the first character of id}
000076c7 7505 jne 0x76ce
000076c9 47 inc di
000076ca e2f7 loop 0x76c3
000076cc eb5e jmp 0x772c
000076ce 8bdf mov bx, di
000076d0 ba0500 mov dx, 0x5
000076d3 8cc8 mov ax, cs
000076d5 8ed8 mov ds, ax
000076d7 be3cf6 mov si, 0xf63c {address of dive string}
000076da b90600 mov cx, 0x6 {"CP3022"} {lenght of string}
000076dd f3a6 rep cmpsb byte [si], [di] {0xf63c} {check string}
000076df 744b je 0x772c {jump if good or continue}
000076e1 ba0500 mov dx, 0x5
000076e4 8bfb mov di, bx
000076e6 be42f6 mov si, 0xf642
000076e9 b90600 mov cx, 0x6 {"CP3024"}
000076ec f3a6 rep cmpsb byte [si], [di] {0xf642}
000076ee 743c je 0x772c
000076f0 8bfb mov di, bx
000076f2 ba0600 mov dx, 0x6
000076f5 be54f6 mov si, 0xf654
000076f8 b90500 mov cx, 0x5 {"CP344"}
000076fb f3a6 rep cmpsb byte [si], [di] {0xf654}
000076fd 742d je 0x772c
000076ff ba0900 mov dx, 0x9
00007702 8bfb mov di, bx
00007704 be48f6 mov si, 0xf648
00007707 b90600 mov cx, 0x6 {"CP3044"}
0000770a f3a6 rep cmpsb byte [si], [di] {0xf648}
0000770c 741e je 0x772c
0000770e 8bfb mov di, bx
00007710 be4ef6 mov si, 0xf64e
00007713 b90600 mov cx, 0x6 {"CP3042"}
00007716 f3a6 rep cmpsb byte [si], [di] {0xf64e}
00007718 7412 je 0x772c
0000771a 8bfb mov di, bx
0000771c ba0700 mov dx, 0x7
0000771f be59f6 mov si, 0xf659
00007722 b90600 mov cx, 0x6 {"CP3104"}
00007725 f3a6 rep cmpsb byte [si], [di] {0xf659}
00007727 7403 je 0x772c
00007729 ba0000 mov dx, 0x0 {fail}
0000772c 1e push ds {var_10_1}
0000772d 50 push ax {var_12_1}
0000772e b84000 mov ax, 0x40
00007731 56 push si {var_14_1}
00007732 bec300 mov si, 0xc3
00007735 8bc2 mov ax, dx
00007737 ab stosw word [di]
00007738 5e pop si {var_14_1}
00007739 58 pop ax {var_12_1}
0000773a 1f pop ds {var_10_1}
0000773b 5f pop di {var_e_1}
0000773c 07 pop es {var_c_1}
0000773d 5e pop si {var_a_1}
0000773e 1f pop ds {var_8_1}
0000773f 59 pop cx {var_6_1}
00007740 5b pop bx {var_4_1}
00007741 e956ff jmp 0x769a
sub_7744:
00007744 1e push ds
00007745 2e8e1ed9c6 mov ds, word [cs:0xffffffffffffc6d9]
0000774a 8a16b200 mov dl, byte [0xb2]
0000774e 32e4 xor ah, ah
00007750 1f pop ds
00007751 c3 retn