17.asm

; differentiate left and right shift keys with scancodes
[org 0x0100]
 jmp start
; keyboard interrupt service routine

kbisr: push ax
 push es
 mov ax, 0xb800
 mov es, ax ; point es to video memory
 in al, 0x60 ; read a char from keyboard port
 cmp al, 0x2a ; is the key left shift
 jne nextcmp ; no, try next comparison
 mov byte [es:0], 'L' ; yes, print L at top left
 jmp nomatch ; leave interrupt routine
nextcmp: cmp al, 0x36 ; is the key right shift
 jne nomatch ; no, leave interrupt routine
 mov byte [es:0], 'R' ; yes, print R at top left
nomatch: mov al, 0x20
 out 0x20, al ; send EOI to PIC
 pop es
 pop ax
 iret
 
 
start: xor ax, ax
 mov es, ax ; point es to IVT base
 cli ; disable interrupts
 mov word [es:9*4], kbisr ; store offset at n*4
 mov [es:9*4+2], cs ; store segment at n*4+2
 sti ; enable interrup
 
 
 l1: jmp l1 ; infinite loop