; Terminate and Stay Resident program that clears screen, changes bgcolor to GREEN when 'G' is pressed, changes bgcolor to Blue when 'B' is pressed.

[org 0x0100]
jmp  start

oldkb:	      dd   0
attrib:       db   0x07


; keyboard interrupt service routine
kbisr:    push ax
          in al, 0x60               ; read char from keyboard port
          cmp al, 0x22              ; has G pressed
          je setGreen

          cmp al, 0x30              ; has B pressed
          je setBlue

          jmp noMatch               ; B or G not pressed

setBlue:  mov byte [cs:attrib], 0x17
jmp clrscr

setGreen: mov byte [cs:attrib], 0x27

clrscr:   mov  ax, 0xb800            ; load video base in ax
          mov  es, ax                ; point es to video base
          xor  di, di                ; point di to top left column
	  mov  al, 0x20              ; load space char in al

          mov  ah, [cs:attrib]       ; load attribute byte in ah
          mov  cx, 2000              ; number of screen locations
          cld                        ; auto increment mode
          rep  stosw                 ; clear the whole screen
jmp exit              
noMatch: pop ax
         jmp far [cs:oldkb]	     ; call original keyboard ISR

exit:    mov al, 0x20
         out 0x20, al 		     ; send EOI to PIC
         pop ax
         iret                        ; return from interrupt


start:    mov  ax, 0xb800            ; load video base in ax
          mov  es, ax                ; point es to video base
          xor  di, di                ; point di to top left column
	  mov  al, 0x20              ; load space char in al

          mov  ah, [cs:attrib]       ; load attribute byte in ah
          mov  cx, 2000              ; number of screen locations
          cld                        ; auto increment mode
          rep  stosw                 ; clear the whole screen

          xor  ax, ax
          mov es, ax                 ; point es to IVT base
          mov ax, [es:9*4]
          mov [oldkb], ax            ; save offset of old routine
          mov ax, [es:9*4+2]
          mov [oldkb+2], ax 	      ; save segment of old routine           
          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 interrupts

          mov  dx, start             ; end of resident portion
          add  dx, 15                ; round up to next para
          mov  cl, 4
          shr  dx, cl                ; number of paras 
          mov  ax, 0x3100            ; terminate and stay resident
          int  0x21