cpu 8086
org 0x0100
  push cs
  pop es
  mov di, isr_dos_main
  mov ax, 0x21
  call intr_register

  int3

  cli
.halt:
  hlt
  jmp .halt

isr_dos_main:
  ; Bail out if subfunction number too high
  cmp ah, 0x0F
  jnc isr_invalid

  ; allocate our return address
  push bx
  ; actual BX backup
  push bx

  ; transfer subfunction number into BX
  xor bx, bx
  mov bl, ah
  ; table offset = AH * 2
  shl bx, 1
  ; fetch from table
  mov bx, [dos_functions+bx]

  ; inject our address into the stack
  add sp, 4
  push bx
  sub sp, 2

  ; restore BX
  pop bx
  ; reads our address from the stack and jumps there
  ret

; ISR for invalid subfunctions or unimplemented
isr_invalid:
  mov bp, 0xFEFE
  int3
.hlt:
  hlt
  jmp .hlt
; ISR tail to set carry flag to signal error
isr_error:
  push bp
  mov bp, sp
  ; set carry flag
  or WORD [SS:BP+6], 1
  pop bp
isr_return:
  iret

; Table for DOS subfunctions
dos_functions:
  ; AH 00-03
  dw isr_invalid
  dw isr_getc_echo
  dw isr_putc
  dw isr_invalid
  ; AH 04-07
  dw isr_invalid
  dw isr_invalid
  dw isr_invalid
  dw isr_invalid
  ; AH 08-0B
  dw isr_invalid
  dw isr_invalid
  dw isr_invalid
  dw isr_invalid
  ; AH 0C-0F
  dw isr_invalid
  dw isr_invalid
  dw isr_invalid
  dw isr_invalid

%include "intr.asm"

%include "chario.asm"

kernel_end: