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More details on the A20 problem with the DeskPro 386 ROM

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This commit is contained in:
Jeff Parsons 2015-04-17 18:58:54 -07:00 • committed by jeffpar
commit 10202ccef5
7 changed files with 1340 additions and 1228 deletions
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32
devices/pc/bios/compaq/deskpro386/1988-01-28.nasm
View file

@ -6628,12 +6628,12 @@ xbc71: call xa3e0 ; 0000BC71 E86CE7 '.l.'
;;
;; Relocate the ROM and initialize conventional RAM
;;
call xc825 ; 0000BCAF E8730B '.s.'
call xc825 ; 0000BCAF E8730B
;;
;; This function loads the GDTR with [00FF0730,0047], but since the previous call
;; turned A20 off, the first selector load crashes, because physical address %FF0730
;; requires A20 on.
;; disabled the A20 line, the first selector load crashes, because physical address
;; %FF0730 requires A20.
;;
call xf480 ; 0000BCB2 E8CB37
@ -8142,7 +8142,7 @@ xc89d: lidt [cs:0xa151] ; 0000C89D 2E0F011E51A1 '....Q.'
;;
;; Disable A20
;;
call xa478 ; 0000C8B4 E8C1DB '...'
call xa478 ; 0000C8B4 E8C1DB
;;
;; Get the (conventional) memory size in Kb from 0x40:0x0013, divide by 64 (0x40) to yield
@ -11574,14 +11574,14 @@ xe770: test dl,0x80 ; 0000E770 F6C280 '...'
jnz xe793 ; 0000E773 751E 'u.'
;;
;; More disk I/O (TODO: Investigate)
;; More diskette I/O (TODO: Investigate)
;;
push bx ; 0000E775 53 'S'
call xd540 ; 0000E776 E8C7ED '...'
pop bx ; 0000E779 5B '['
;;
;; Check the ROM progress byte for 0xBD (value I've seen: 0xB9)
;; Check the ROM status byte for 0xBD (values I've seen: 0xB9)
;;
in al,0x84 ; 0000E77A E484 '..'
cmp al,0xbd ; 0000E77C 3CBD '<.'
@ -11589,14 +11589,15 @@ xe770: test dl,0x80 ; 0000E770 F6C280 '...'
;;
;; This code looks broken: ES:BX points to the boot sector just loaded
;; (0x0000:0x7C00), and apparently it wants to scan the first 9 words of
;; the boot sector, to see if they all match the 1st word; if they do,
;; (0x0000:0x7C00), and apparently the ROM wants to scan the first 9 words
;; of the boot sector, to see if they all match the 1st word; if they do,
;; then it's likely the boot sector is invalid (hence the "boot_error"
;; message). However, it's loading (AX) with the 1st word from DS:BX
;; (0x0040:0x7C00) rather than ES:BX, which increases the likelihood that
;; this code will always see a difference and never trigger the error.
;; message). However, it's loading AX with the 1st word from DS:BX
;; (0x0040:0x7C00) rather than ES:BX, which aside from being the wrong word,
;; also increases the likelihood that this code will always see a difference
;; and never trigger the error.
;;
;; The value in (AX) is typically 0x0000.
;; The value in AX is typically 0x0000.
;;
mov ax,[bx] ; 0000E780 8B07 '..'
mov cx,0x9 ; 0000E782 B90900 '...'
@ -12986,10 +12987,11 @@ xf494: mov al,0x0 ; 0000F494 B000
;; And even if we DID access it from the "low" address, it contains base addresses (eg,
;; for selector 0x28) located at %FFxxxx, so we're still screwed if A20 is disabled.
;;
;; TODO: Determine how this code works in "real life"
;; TODO: Determine how this code worked in "real life"
;;
;; FYI, it seems this code doesn't do anything PROVIDED bits 6 and 7 of the RAM Settings
;; register are set to anything other than 0x40.
;; FYI, it seems this code doesn't do anything if bits 6 and 7 of the RAM Settings
;; register are set to anything other than 0x40 (ie, it returns to real-mode almost
;; immediately after entering protected-mode).
;;
lgdt [cs:0x077e] ; 0000F498 load [gdtr_hi] into GDTR
mov eax,cr0 ; 0000F49E 0F2000

2
devices/pc/bios/compaq/deskpro386/README.md
View file

@ -63,7 +63,7 @@ while the /PGM should be connected to INPUT HIGH VOLTAGE. So I wired pin 27 (/P
worked perfectly. The NYC Resistor article implied that every *active low* pin should be connected to GND, but apparently
there are exceptions to that general rule.
Producing BIOS Source Code
Producing ROM Source Code
---
In the current directory, an original ROM can be regenerated from the JSON-encoded file: