Revision as of 15:01, 16 January 2016 view source osdev>Catnikita255 →‎Keyboard Controller ← Older edit		Revision as of 11:25, 23 July 2016 view source osdev>New16 reworked section "INT 15, 2401" Newer edit →
Line 191: However, the Fast A20 method is not supported everywhere and there is no reliable way to tell if it will have some effect or not on a given system. Even worse, on some systems, it may actually do something else like blanking the screen, so it should be used only after the [[BIOS]] has reported that FAST A20 is available. Code for systems lacking FAST A20 support is also needed, so relying only on this method is discouraged. Also, on some chipsets you might have to enable Fast A20 support in the BIOS configuration screen. ===INT 15~~, 2401~~=== Another way is to use the BIOS. <source lang="asm"> ;FASM ~~mov ax, 0x2401~~ use16 ~~int 0x15~~ mov ax,2403h ;--- A20-Gate Support --- int 15h jb a20_ns ;INT 15h is not supported cmp ah,0 jnz a20_ns ;INT 15h is not supported mov ax,2402h ;--- A20-Gate Status --- int 15h jb a20_failed ;couldn't get status cmp ah,0 jnz a20_failed ;couldn't get status cmp al,1 jz a20_activated ;A20 is already activated mov ax,2401h ;--- A20-Gate Activate --- int 15h jb a20_failed ;couldn't activate the gate cmp ah,0 jnz a20_failed ;couldn't activate the gate a20_activated: ;go on </source> If the one interrupt failes you will have to use another method. (See below.) If there is an error, the CF will be set, and the error code will be in AH. On success, the CF is clear, and AH==0. Specifically, AH==0x86 means the function is not supported, and you will have to use another method. ===Recommended Method===

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