User:Johnburger/Demo/Boot/A20

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The 8086 has a 20-bit address bus. If you loaded a segment register with 0FFFFh and accessed 0FFFFh:0FFFFh, the Segment:Offset addition would wrap and you'd actually be accessing 0000_FFEFh. The BIOS would "take advantage" of this quirk and use it to access both top-of-Address ROM and botttom-of-Address RAM with the same segment register.

Of course, then the '286 PC/AT came out with a 24-bit bus - and the wrap no longer occurred. You could now use it to access the (nearly) 64K of High Memory while still in Real Mode! But what about all the legacy code that used the trick? That was now broken.

To "fix" this, the designers of the PC/AT added a Gate to the A20 line, to hold A20 on the memory bus at zero regardless of what the CPU wanted. This emulated the wrap-around "feature", but made the high memory inaccessible.

So to fix that, they found a spare output pin that they could program at run-time to turn the Gate on or off. Where was this spare pin? The keyboard controller, of course...

As PCs became more sophisticated, the A20 Gate stayed, but the programming mechanism to turn it on or off changed - and it wasn't really standardised. Since this program wants to access high memory, it has to turn off the Gate - and it uses a number of techniques to attempt this, as recommended by this page in the Wiki.

Demo/Boot/A20.inc

;
; Boot/A20.inc
;
 
; The 8086 has a 20-bit address bus. If you loaded a segment register with
; 0FFFFh and accessed 0FFFFh:0FFFFh, the Segment:Offset addition would wrap
; and you'd actually be accessing 0000_FFEFh. The BIOS would "take advantage"
; of this quirk and use it to access both top-of-Address BIOS and botttom-of-
; Address RAM with the same segment register.
;
; Of course, then the '286 PC/AT came out with a 24-bit bus - and the wrap no
; longer occurred. You could use it to access the (nearly) 64K of High Memory!
; But what about all the legacy code? That was now broken.
;
; To "fix" this, the designers of the PC/AT added a Gate to the A20 line, to
; hold A20 on the memory bus at zero regardless of what the CPU wanted. This
; emulated the wrap-around "feature", but made the high memory inaccessible.
;
; So to fix that, they found a spare output that they could program at run-time
; to turn the Gate on or off. Where was this spare pin? The keyboard controller,
; of course...
;
; As PCs became more sophisticated, the A20 Gate stayed, but the programming
; mechanism to turn it on or off changed - and it wasn't really standardised.
; Since this program wants to access high memory, it has to turn off the Gate -
; and it uses a number of techniques to attempt this.
;
; Of course, this means finding out whether A20 is off, both up front and after
; each technique. How? Go to a known memory location, get its value, and compare
; it with its high-memory "alias". If they're different, then the Gate is off.
; If they're the same, then maybe it was a coincidence? Change the original
; location, and check the alias again. If they're now different (they were the
; same, but now theyre different), the Gate must be off.
 
A20.Enable EQU             (BIOS.A20.Fn << 8) | BIOS.A20.Enable
 
; [0000h:BIOS.Entry + BIOS.Sig.Pos] holds the value BIOS.Sig.Value
; [FFFFh:BIOS.Entry + BIOS.Sig.Pos + 10h] is its alias, if the A20 Gate is on.
Sig.Alias  EQU             BIOS.Entry + BIOS.Sig.Pos + 10h
 
Boot.A20:
; CS:BIOS.Sig.Pos has a known, safely-modifable value.
; Prepare for testing, by comparing CS:BIOS.Sig.Pos with 0FFFFh:Sig.Alias
                MOV             SI,BIOS.Sig.Pos
 
                MOV             AX,0FFFFh       ; End of 1 MB
                MOV             ES,AX           ; In ES
                MOV             DI,Sig.Alias
 
; First things first: has it already been done for us?
                CALL            .Test           ; Test if A20 already on
                JE              .End            ; Yep! Nothing to do
 
; No. OK, maybe the BIOS can help.
                MOV             AX,A20.Enable
                INT             BIOS.A20.Int    ; Using BIOS
 
                CALL            .Test           ; Did it work?
                JE              .End            ; Yep! Nothing more to do
 
; No. OK: the traditional ways are always the best?
                CALL            Boot.A20.Switch ; Switch A20 myself
;               JNZ             $               ; Uh oh! ***STOP!***
 
                CALL            .Test           ; Did it work?
                JE              .End            ; Yep! Nothing more to do
 
; No. Maybe it's a PS/2?
                IN              AL,Dev.A20.Fast.Port ; Try Fast A20
                TEST            AL,Dev.A20.Fast.A20 ; Enabled?
                JNZ             $               ; Uh oh! Isn't working! ***STOP!***
                OR              AL,Dev.A20.Fast.A20    ; Enable it
                AND             AL,~Dev.A20.Fast.Reset ; But NOT Fast Reset!
                OUT             Dev.A20.Fast.Port,AL
 
                CALL            .Test           ; Did it work?
                JE              .End            ; Yep! Finally!
 
                JMP             $               ; No reaction! ***STOP!***
Boot.A20.Test:
                MOV             DX,[CS:SI]      ; Get current value
                CMP             DX,[ES:DI]      ; Same at end of RAM?
                JNE             .Good           ; No, so no problem!
 
                NOT     WORD    [CS:SI]         ; Change current value
                CMP             DX,[ES:DI]      ; Still same as old?
                JMP             .Bad            ; No, so problem!
.Good:
                XOR             AX,AX           ; Set Z flag
.Bad:
                RET
 
;-------------------------------------------------------------------------------
 
Boot.A20.Switch:
                CALL            .Wait          ; Wait for 8042 to be ready
                JNZ             .Dead          ; Uh oh...
 
                MOV             AL,Dev.A20.Ctrl.Write
                OUT             Dev.A20.Ctrl.Port,AL ; Tell 8042 we're writing to it
                CALL            .Wait          ; Wait for acknowledge
                JNZ             .Dead          ; Uh oh...
 
                MOV             AL,Dev.A20.Data.Enable
                OUT             Dev.A20.Data.Port,AL ; Tell 8042 to Enable A20
;...............................................................................
.Wait:
                XOR             CX,CX           ; Try a LARGE number of times!
.Loop:
                IN              AL,Dev.A20.Stat.Port ; Wait to write
                TEST            AL,Dev.A20.Stat.InFull ; YES, In! It's backwards
                LOOPNZ          .Loop           ; Nope. Still full
.Dead:
                RET
 
;-------------------------------------------------------------------------------
 
Boot.A20.End:
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