1 files changed, 157 insertions, 0 deletions
diff --git a/src/boot/stage2_a20.asm b/src/boot/stage2_a20.asm
new file mode 100644
index 0000000..c0951ad
--- /dev/null
+++ b/src/boot/stage2_a20.asm
@@ -0,0 +1,157 @@
+; functions to handle A20 address line status and switching
+
+; returns 0 if not A20_ENABLED, 1 if A20_ENABLED in AX
+check_A20_enabled:
+	pushf
+	push ds
+	push es
+	push di
+	push si
+	
+	cli
+	
+	xor ax, ax
+	mov es, ax
+	mov di, 0x0500			; es:di = 0000:0500
+	
+	mov ax, 0xffff
+	mov ds, ax
+	mov si, 0x0510			; ds:si = ffff:0510
+	
+	mov al, byte [es:di]		; preserve values in memory
+	push ax				; on stack
+	mov al, byte [ds:si]
+	push ax
+	
+	mov byte [es:di], 0x00		; now the test: write 0x00 to 0000:0500
+	mov byte [ds:si], 0xFF		; write 0xff to ffff:0510
+	
+	cmp byte [es:di], 0xFF		; memory wrap? A20 notA20_ENABLED
+	
+	pop ax				; restore original memory contents
+	mov byte [ds:si], al
+	pop ax
+	mov byte [es:di], al
+	
+	je .enabled
+	jmp .disabled
+	
+.enabled:
+	mov al, '+'
+	call print_char
+	mov ax, 1			; not wrapped around
+	jmp .exit
+
+.disabled:
+	mov al, '-'
+	call print_char
+	mov ax, 0			; wrapped around (last cmp)
+	jmp .exit
+	
+.exit:
+	pop si
+	pop di
+	pop es
+	pop ds
+	popf
+
+	ret
+
+check_and_enable_A20:
+	call check_A20_enabled
+	cmp ax, 1
+	je A20_ENABLED
+
+A20_ENABLE_KBD_PORT:
+	mov al, 'K'
+	call print_char
+	mov al, 0xdd
+	out 0x64, al
+
+	call check_A20_enabled
+	cmp ax, 1
+	je A20_ENABLED
+
+A20_ENABLE_KBD_OUT:
+
+	mov al, 'k'
+	call print_char
+
+	cli			; disable interrupts, we talk directly to the keyboard ports
+
+        call    .wait_input
+        mov     al,0xAD
+        out     0x64,al		; disable keyboard
+        call    .wait_input
+
+        mov     al,0xD0
+        out     0x64,al		; tell controller to read output port
+        call    .wait_output
+
+        in      al,0x60
+        push    eax		; get output port data and store it
+        call    .wait_input
+
+        mov     al,0xD1
+        out     0x64,al		; tell controller to write output port
+        call    .wait_input
+
+        pop     eax
+        or      al,2		; set bit 1 (enable a20)
+        out     0x60,al		; write out data back to the output port
+
+        call    .wait_input
+        mov     al,0xAE		; enable keyboard
+        out     0x64,al
+
+        call    .wait_input
+
+        sti
+
+        jmp .retest
+
+; wait for input buffer to be clear
+.wait_input:
+        in      al,0x64
+        test    al,2
+        jnz     .wait_input
+        ret
+
+; wait for output buffer to be clear
+.wait_output:
+        in      al,0x64
+        test    al,1
+        jz      .wait_output
+        ret
+
+.retest:
+	call check_A20_enabled
+	cmp ax, 1
+	je A20_ENABLED
+
+A20_FAST_SPECIAL_PORT:
+
+	mov al, 'F'
+	call print_char
+
+	in al, 0x92
+	or al, 2
+	out 0x92, al
+
+	call check_A20_enabled
+	cmp ax, 1
+	je A20_ENABLED
+	ret
+
+A20_ENABLE_VIA_BIOS:
+	mov al, 'B'
+	call print_char
+	mov ax, 0x2401
+	int 0x15
+
+	call check_A20_enabled
+	cmp ax, 1
+	je A20_ENABLED
+  	
+A20_ENABLED:
+	ret