Retro/pcjs
Watch
1
0
Fork
You've already forked pcjs
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Blog post update

Browse source
This commit is contained in:
Jeff Parsons 2016-01-21 22:54:47 -08:00
commit 8eb1e0aa21
2 changed files with 19 additions and 19 deletions
Download patch file Download diff file Expand all files Collapse all files

32
_posts/2015-04-16-compaq-deskpro-386-update.md
View file

@ -111,22 +111,22 @@ three physical address ranges at any time, regardless whether A20 is disabled or
Which is a good thing, because I came across at least one code sequence in the ROM BIOS that enters
protected-mode with A20 disabled -- an unwise thing to do on most machines:
;;
;; 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
;; immediately after entering protected-mode).
;;
lgdt [cs:0x077e] ; 0000F498 load [gdtr_hi] into GDTR
mov eax,cr0 ; 0000F49E 0F2000
or ax,0x1 ; 0000F4A1 0D0100
mov cr0,eax ; 0000F4A4 0F2200
jmp 0x28:xf4ac ; 0000F4A7 EAACF42800
;
; 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
; immediately after entering protected-mode).
;
lgdt [cs:0x077e] ; 0000F498 2E0F01167E07; load [gdtr_hi] into GDTR
mov eax,cr0 ; 0000F49E 0F2000
or ax,0x1 ; 0000F4A1 0D0100
mov cr0,eax ; 0000F4A4 0F2200
jmp 0x28:xf4ac ; 0000F4A7 EAACF42800
Before fully understanding the DeskPro's unusual A20 management, PCjs worked around it by
redirecting all A20 changes from the Bus component to the CPU component, giving the CPU first