Protected Mode

main.asm

;---
;		Giza system bootstrap
;		- check for a supported CPU, exit with error message if unsupported
;		- Enter Protected Mode
;		- Call system bootstrap
;		- Leave Protected Mode
;		- exit (status = byte obtatained)
;
Unsupported db      "CPU: Not an 80386-class CPU; unable to start",0Dh,0Ah,'$'
AlreadyInPM db      "Already in Protected Mode!",0Dh,0Ah,'$'
A20LineFail db      "A20: Unable to enter protected mode",0Dh,0Ah,'$'

start:		call	GzIsSupportedCPU		; make sure having proper CPU and mode
			add		al, al
			jz		successful				; OK.
			lea		dx, A20LineFail
			jc		GzPrintErr
			lea		dx, Unsupported
			js		GzPrintErr
			lea		dx, AlreadyInPM
			
GzPrintErr:	push	cs						; Print error message and exit
			pop		ds
			mov		ah, 9
			int		21h
			mov		ax, 4cffh				; exit, status=-1
			int		21h
			
successful:	call    GzSysInitStartup
			mov     ah, 4ch					; exit
			mov     al, cl					; status = the byte
			int     21h

codeseg		ends
			end     start

startup.asm

;	
;	+---------------------------------------------------------------------------+
;   |	GzSysInitStartup  - Enters protected mode, entering the Protected		|
;	|						Mode section of the bootstrap executable.			|
;	+---------------------------------------------------------------------------+
;	
GzStartBaseSystem 	proc								; ===================================================== 
														; === Enter PM and start up =========================== 
					pushf								; ===================================================== 
					push    ds							; === Set the processor into Protected Mode and FAR ===
														; === JMP to the startup code in the system loader. ===
														; === From there,  we load both the ELF kernel and  ===
														; === system libraries. This allows us to both load ===
														; === and start Human Experience which will present ===
														; === the user with a welcome prompt and allow them ===
														; === to sign in.									===
														; =====================================================
					mov     ax, cs						; 
					movzx   eax, ax						; ===
					shl     eax, 4						; eax = base (code segment)
					mov     dword ptr codeseg[2], eax	; 
					mov     byte ptr codeseg[5], 9Ah	; set segment attribute
					mov     dx, offset nulldes			; 
					movzx   edx, dx						; 
					add     eax, edx     			    ; eax = GDT base
					push    eax							; 
					push    20h							; 
					movzx   eax, sp          			; 
					cli        				            ; make sure no ISR will interfere now
					lgdt    fword ptr ss:[eax]			; LGDT is necessary before switch to PM
					add     sp, 6						; 
					mov     eax, cr0					; 
					or      al, 1						; 
					mov     cr0, eax					; sets Protected Mode
					db      0eah						; far jump to set CS & clear prefetch queue
					dw      GzStartupMain				; 
					dw      8							; 
														; === Returns us back to RM =====================
					mov     cr0, eax					; 
					db      0eah   						; far jump to restore CS & clear prefetch queue
					dd      LeaveProtectedMode 			; it MUST be a far jump - return crashes!
LeaveProtectedMode:	pop     ds
					popf
					ret
GzSysInitStartup 	endp

verify.asm

;---
;       GzIsSupportedCPU  - make sure have proper CPU, in Real Mode, and A20 enabled
;
;       changes AX,DX
;       returns AL=error code if anything wrong
;       error codes:    040h    - not 32-bit CPU (PM,A20 not tested)
;                       001h    - already in PM (386+, A20 not tested)
;                       0FFh    - A20 disabled (32-bit CPU in RM)
;
GzIsSupportedCPU	proc
					pushf   									; save flags
					cli     									; make sure no ISR will interfere now
																; 
					pushf										; 1. make sure have at least 386
					pop     ax          						; AX=flags
					xor     ah, 40h      						; toggle NT
					push    ax          						; stack: modified_flags,original_flags
					popf										; 
					pushf										; stack: modified_flags_from_cpu,original_flags
					pop     dx									; DX=flags passed via CPU
					mov     al, dh								; 
					xor     al, ah								; 
					jnz     NotSupportedCPU   					; improper CPU
																; 
					smsw    ax									; 2. make sure are in Real Mode
					and     al, 1								; 
					jnz     NotSupportedCPU						; if already in PM (maybe VM86)
																; 
					push    ds          	    				; 3. make sure A20 is enabled
					push    es									; 
					push    bx									; 
					xor     bx, bx								; 
					mov     ds, bx								; 
					mov     ax, -1								; 
					mov     es, ax								; 
					xor     [bx], ah							; change byte[000000h]
					mov     al, es:[bx + 10h]					; get byte[100000h]
					xor     [bx], ah							; change byte[000000h]
					xor     al,es:[bx + 10h]					; compare byte[100000h] with its previous value
																;  0 if unchanged, -1 means [000000h]==[100000h]
					pop     bx									; 
					pop     es									; 
					pop     ds									; 
																; 
NotSupportedCPU:	popf										; restore flags
					test    al, al								; test error code
					ret
GzIsSupportedCPU  endp