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author | Andreas Baumann <mail@andreasbaumann.cc> | 2022-09-02 09:18:52 +0200 |
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committer | Andreas Baumann <mail@andreasbaumann.cc> | 2022-09-02 09:18:52 +0200 |
commit | 52be99d8c0862ff87db9a4f9ccec1ac4b5f7caed (patch) | |
tree | 1a1a1c98090afd8459cc8f045f9de2d4a7cba5ab /floppy/tests/test_a20.asm | |
parent | 15adaba9eaa6a98c8b55bc5c5f73c3a9e0e55e7a (diff) |
-rw-r--r-- | floppy/tests/test_a20.asm | 61 |
diff --git a/floppy/tests/test_a20.asm b/floppy/tests/test_a20.asm new file mode 100644 index 0000000..43da6e3 --- /dev/null +++ b/floppy/tests/test_a20.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 |