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Updated information on A20 handling in DeskPro 386 systems

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This commit is contained in:
Jeff Parsons 2015-04-19 11:58:54 -07:00 • committed by jeffpar
commit c684371852
2 changed files with 67 additions and 87 deletions
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23
devices/pc/bios/compaq/deskpro386/1988-01-28.nasm
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@ -28,8 +28,8 @@
; replacing the ROM in the first megabyte with write-protected RAM; the top 64Kb of that
; RAM must first be initialized with the 64Kb at %000F0000 prior to remapping. It's also
; possible to copy external ROMs from %000C0000 through %000EFFFF into the bottom 64Kb of
; that RAM, but this is only done for ROMs known to contain relocatable code (eg, a Compaq
; Enhanced Video Graphics card).
; that RAM, but this is only done for ROMs known to contain relocatable code; eg, a Compaq
; Video Graphics Controller (VGC) Board.
;
; Every DeskPro 386 system must have a MINIMUM of 1Mb of RAM, of which either 256Kb,
; 512Kb, or 640Kb can be physically mapped as conventional memory (at the bottom of the
@ -6636,14 +6636,10 @@ xbc71: call xa3e0 ; 0000BC71 E86CE7 '.l.'
;;
call xc825 ; 0000BCAF E8730B
;;
;; The next function loads the GDTR with [00FF0730,0047], but since the previous call
;; disabled the A20 line, the first selector load crashes, because physical address
;; %FF0730 requires A20.
;;
call xf480 ; 0000BCB2 E8CB37
call xe7df ; 0000BCB5 E8272B '.',0x27,'+'
call xe7df ; 0000BCB5 E8272B
sti ; 0000BCB8 FB '.'
mov al,0x21 ; 0000BCB9 B021 '.!'
out 0x84,al ; 0000BCBB E684 '..'
@ -12988,12 +12984,11 @@ xf494: mov al,0x0 ; 0000F494 B000
out 0x80,al ; 0000F496 E680
;;
;; When we arrive here, the A20 line has been disabled, so in theory, the GDT is
;; accessible only at the "low" ROM address (%0F0730), not the "high" address (%FF0730).
;; 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 worked in "real life"
;; When we arrive here, the A20 line has been disabled; on most systems, that would
;; mean that the ROM's GDT would only be accessible at the "low" ROM address (%0F0730),
;; not the "high" address (%FF0730). But fortunately, A20 management on Compaq
;; DeskPros affects wrap-around only from the 1st to the 2nd megabyte; no other address
;; range is affected.
;;
;; 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