ref: 46bac13b16df5cbd92ea1da12cfef168201f0dad
dir: /sys/src/boot/pc/a20.s/
#include "x16.h" #undef ORB TEXT a20test(SB), $0 LONG $1234567 TEXT a20check(SB), $0 MOVL $10000, CX _loop: LEAL a20test(SB), AX MOVL (AX), BX ADDL $12345, BX MOVL BX, (AX) ORL $(1<<20), AX MOVL (AX), AX CMPL AX, BX JNZ _done LOOP _loop RET _done: /* return directly to caller of a20() */ ADDL $4, SP XORL AX, AX RET TEXT a20(SB), $0 CALL a20check(SB) /* try bios */ CALL rmode16(SB) STI LWI(0x2401, rAX) BIOSCALL(0x15) CALL16(pmode32(SB)) CALL a20check(SB) /* try fast a20 */ MOVL $0x92, DX INB TESTB $2, AL JNZ _no92 ORB $2, AL ANDB $0xfe, AL OUTB _no92: CALL a20check(SB) /* try keyboard */ CALL kbdempty(SB) MOVL $0x64, DX MOVB $0xd1, AL /* command write */ OUTB CALL kbdempty(SB) MOVL $0x60, DX MOVB $0xdf, AL /* a20 on */ OUTB CALL kbdempty(SB) MOVL $0x64, DX MOVB $0xff, AL /* magic */ OUTB CALL kbdempty(SB) CALL a20check(SB) /* fail */ XORL AX, AX DECL AX RET TEXT kbdempty(SB), $0 _kbdwait: MOVL $0x64, DX INB TESTB $1, AL JZ _kbdempty MOVL $0x60, DX INB JMP _kbdwait _kbdempty: TESTB $2, AL JNZ _kbdwait RET