🚀 go-pugleaf

Hello everybody,
		What follows is my idea of all that is necessary to switch
a 486 (or 386 for that matter) into "Flat" protected-mode.
		
		IT does NOT work.

		I would be *most* grateful if anyone could take a look at it
and give me some idea what I'm doing wrong.  It simply hangs the machine, and
the top left-hand corner of the screen remains un-changed.

		I'm using MASM 6, but this should work on most versions of
most assemblers that accept protected mode instructions.

	Alternativly, if anyone has a tiny .EXE or .COM program to (1) switch
the CPU into protected mode, and (2) execute some trivial instructions in an
endless loop, I'd love to get a copy.  I'm much better with .ASM when I've got
some working code to disect.

		All help grately appreciated.

Chris.
sir@usage.csd.unsw.oz.au




PAGE         ,132                        ;Wide output
TITLE        Empty .EXE skeleton for protected mode
.RADIX       16                          ;Numbers I type in are HEX by default
INCLUDE      MACROS.ASM                  ;My pre-built functions
SKEL         segment para public 'CODE'  ;Where the stuff will be starting from
             public START
             assume CS:SKEL, DS:DATA, ES:DATA, SS:STACK
.486p        ;Allow protected-mode and 486 instructions.

;zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

PE_FLAG      equ     1
data_selc    equ     20h

;zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

CLR     macro reg    ;This is here because I'm a 6809 veteran and
        sub reg,reg  ;It's a beautiful macro, so why not ? (sub faster than xor)
        endm

;zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

DATA         segment para public 'DATA'  ;Will be placed directly after the CODE
gdt_desc     qword ?
gdt_tab      dw 0,0,0,0
             dw 0FFFFh,0,9200h,8Fh
             dq 0
Intro        db      'Switching into Linear protected-mode now.$'
DATA         ends

;zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

STACK_SIZE   equ     1000h
STACK        segment para stack 'STACK'  ;Will be placed directly after the DATA
             db      (STACK_SIZE) dup(?)
STACK        ends



;zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz
START        proc near               ;Everything starts from here.
PSP_SEG equ  24h
ENV_SEG equ  2ch

             STI                     ;enable interrupts
             MOV     AX,DATA         ;Get the right segments pointing to
             MOV     DS,AX           ;the right places
             MOV     ES,AX

             CLR     AX
             CLI     ;Disable interrupts
             OUT     0A0h,AL         ;Disable NMI too

             mov     dx,offset Intro
             mov     ah,9
             int     21h

;             MOV     EAX,OFFSET GDT_DESC
             mov     eax,offset gdt_tab
             AND     EAX,0FFFFh
             LGDT    fword ptr [EAX]

             mov     ax,bx
             or      al,pe_flag
             lmsw    ax
             clr     eax
             mov     al,data_selc
             mov     ds,ax
             mov     ss,ax
             mov     es,ax
             mov     fs,ax
             mov     gs,ax

             jmp     far ptr c_startup

             ;Exit to DOS when complete (obviously never called)
             MOV AH,4CH
             INT 21H
             RET

c_startup:   mov     esp,eax

             mov     eax,0b8000h
             mov     edi,eax

             inc     dword ptr[edi]

             MOV     AL,1
             OUT     0A0h,AL
             STI

@@:          inc     dword ptr[edi]

             jmp     @B
             ;etc
             hlt


START        endp
;zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

SKEL         ends
             end     START


Chris Drake.
sir@usage.csd.unsw.oz.au