Initial commit

1 files changed, 75 insertions, 0 deletions

diff --git a/mem.asm b/mem.asm
new file mode 100644
index 0000000..c99660e
--- /dev/null
+++ b/mem.asm
@@ -0,0 +1,75 @@
+%ifndef __MEM_ASM
+%define __MEM_ASM
+
+check_a20:
+	pushf				; preserve flags
+	push ds				; data segment
+	push es				; extra segment
+	push di				; destination index
+	push si				; source index
+
+	cli				; disable interrupts
+
+	xor ax, ax			; zero out ax
+	mov es, ax			; zero out extra segment
+
+	not ax				; flip all bits in ax
+	mov ds, ax
+
+	mov di, 0x0500			; destination address
+	mov si, 0x0510			; source address
+
+	mov al, byte [es:di]		; copy byte from destination (FFFF:0500)
+	push ax
+
+	mov al, byte [ds:si]		; copy byte from source (FFFF:0510)
+	push ax
+
+	mov byte [es:di], 0x00		; set es:di to 0
+	mov byte [ds:si], 0xFF		; set es:si to 255
+
+	cmp byte [es:di], 0xFF		; did it wrap around?
+
+	pop ax
+	mov byte [ds:si], al
+
+	pop ax
+	mov byte [es:di], al
+
+	mov ax, 0
+	je check_a20__exit
+
+	mov ax, 1
+
+check_a20__exit:
+	sti				; enable interrupts
+	pop si				; restore registers
+	pop di
+	pop es
+	pop ds
+	popf				; restore flags
+
+	ret
+
+
+enable_a20:				; fast A20
+	cli				; disble interrupts
+	call check_a20			; is A20 already enabled?
+	cmp al, 1
+	je .no_a20
+
+	in al, 0x92			; enable A20
+	test al, 2
+	jnz .return
+	or al, 2
+	and al, 0xFE
+	out 0x92, al
+	jmp .return
+.no_a20:
+	stc				; set carry on failure
+.return:
+	sti				; enable interrupts
+	clc				; clear carry
+	ret
+
+%endif