experimenting with a20 gate

1 files changed, 61 insertions, 0 deletions

diff --git a/boot/check_a20_16.asm b/boot/check_a20_16.asm
new file mode 100644
index 0000000..43da6e3
--- /dev/null
+++ b/boot/check_a20_16.asm
@@ -0,0 +1,61 @@
+; The following code is public domain licensed
+ 
+[bits 16]
+ 
+; Function: check_a20
+;
+; Purpose: to check the status of the a20 line in a completely self-contained state-preserving way.
+;          The function can be modified as necessary by removing push's at the beginning and their
+;          respective pop's at the end if complete self-containment is not required.
+;
+; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
+;          1 in ax if the a20 line is enabled (memory does not wrap around)
+ 
+check_a20:
+    pushf
+    push ds
+    push es
+    push di
+    push si
+ 
+    cli
+ 
+    xor ax, ax ; ax = 0
+    mov es, ax
+ 
+    not ax ; ax = 0xFFFF
+    mov ds, ax
+ 
+    mov di, 0x0500
+    mov si, 0x0510
+ 
+    mov al, byte [es:di]
+    push ax
+ 
+    mov al, byte [ds:si]
+    push ax
+ 
+    mov byte [es:di], 0x00
+    mov byte [ds:si], 0xFF
+ 
+    cmp byte [es:di], 0xFF
+ 
+    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:
+    pop si
+    pop di
+    pop es
+    pop ds
+    popf
+ 
+    ret