Work on interrupt subfunctions, remove drvtab experiments

Makefile

@@ -26,7 +26,7 @@ endif
 default: kernel.rom
 
 kernel.com: kernel/*.asm lib/*.inc
-	nasm -s -o $@ -I lib -I kernel kernel/main.asm
+	nasm -s -o $@ -l kernel.lst -I lib -I kernel kernel/main.asm
 
 debug.rom: debug/*.asm lib/*.inc
 	nasm -s -o $@ -I lib -I debug debug/main.asm && scripts/fix-rom.sh $@

kernel/chario.asm

@@ -1,18 +1,35 @@
-putc:
-  pushf
-  push cs
-  call isr_putc
-  ret
+; Reads a single character with echo
+; out	AL	character
+isr_getc_echo:
+  xor ax, ax
+  int 0x16
+  test al, al
+  jz isr_getc_echo
+  call putc
+  iret
 
+; Reads a single character without echo
+; out   AL      character
 isr_getc:
   xor ax, ax
   int 0x16
   test al, al
   jz isr_getc
+  iret
+
+; Prints a single character
+; in	DL	character
 isr_putc:
+  push ax
+  mov al, dl
+  call putc
+  pop ax
+  iret
+
+putc:
   push bx
   mov ah, 0x0e
   mov bx, 0x0000
   int 0x10
   pop bx
-  iret
+  ret

kernel/drvtab.asm

@@ -1,51 +0,0 @@
-; Drive table
-; Header:
-;	byte	number of drives
-; Item (type BIOS CHS):
-;	byte	type (1)
-;	byte	drive number
-;	word	number of cylinders
-;	byte	number of heads
-;	word	number of sectors per cylinder
-;	3 bytes	padding
-; Item (type BIOS LBA):
-;	byte	type (2)
-;	byte	drive number
-;	dword	offset
-;	dword	size
-
-; Create a drive table at ES:DI
-drvtab_create:
-  mov ax, 0x2B
-  call intr_register
-
-  mov al, 8  ; 8 drives per default
-  mov ah, 10 ; 10 bytes per drive
-  stosb
-  mul ah
-
-  xor cx, cx
-  xchg cx, ax
-  rep stosb
-
-  ret
-
-; Get drive table item
-; IN	DL	drive number (A=0, B=1, ...)
-; OUT	DS:SI	ptr to drive struct
-drvtab_load:
-  push ax
-  mov ax, 0x2B
-  call intr_load
-  cmp dl, [ds:si]
-  jnb .err
-  mov al, 10
-  mul dl
-  inc ax
-  add si, ax
-  pop ax
-  ret
-.err:
-  pop ax
-  stc
-  ret

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: