Work on interrupt subfunctions, remove drvtab experiments

kernel/main.asm

@@ -6,10 +6,7 @@ org 0x0100
   mov ax, 0x21
   call intr_register
 
-.loop:
-  mov ah, 0x01
-  int 0x21
-  jmp .loop
+  int3
 
   cli
 .halt:
@@ -17,10 +14,41 @@ org 0x0100
   jmp .halt
 
 isr_dos_main:
-  cmp ah, 0x01
-  je isr_getc
-  cmp ah, 0x02
-  je isr_putc
+  ; 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
@@ -30,9 +58,31 @@ isr_error:
 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 "drvtab.asm"
 
 %include "chario.asm"
 
-times 1000 db 0xEA
+kernel_end: