Make dprintf the default debugging facility

Makefile

@@ -18,7 +18,7 @@ QEMU_ARGS += --enable-kvm
 endif
 
 ifdef DEBUG
-ROMS += debug.rom
+ROMS += dprintf.rom
 NASM_ARGS += -l $(basename $@).lst
 endif
 
@@ -65,7 +65,7 @@ fate.bs: boot/fat.asm
 	$(NASM) $(NASM_ARGS) -DFAT16 -DLBA -DLARGE -o $@ $<
 
 # BIOS option roms
-%.rom: rom/%.asm %/*.asm utils/fix-rom
+%.rom: rom/%.asm utils/fix-rom
 	$(NASM) $(NASM_ARGS) -o $@ $< && utils/fix-rom $@
 
 fdimage.img: fat1.bs $(DISTFILES)

rom/dprintf.asm

@@ -0,0 +1,159 @@
+	cpu 8086
+	org 0x0000
+
+	db 0x55, 0xAA
+	db 0x00
+	jmp init
+	nop
+
+	times (0x18 - ($-$$)) db 0
+	dw 0
+	dw pnp
+
+pnp:	db "$PnP"
+	db 1 ; version 1
+	db 2 ; 2 * 16 length
+	dw 0 ; offset of next header
+	db 0
+	db 0 ; checksum (filled by fix-rom)
+	dd 0 ; device identifier
+	dw 0 ; manufacturer string
+	dw name ; product name string
+	db 0,0,0 ; device type string
+	db 0x20 ; device indicator, bit for "read cacheable" set
+	dw 0 ; boot connection vector
+	dw 0 ; boot disconnect vector
+	dw 0 ; bootstrap entry point
+	dw 0 ; reserved
+	dw 0
+
+name:	db "dprintf"
+
+init:	push ds
+	push ax
+	xor ax, ax
+	mov ds, ax
+
+	mov word [1*4], int3entry
+	mov word [1*4+2], cs
+	mov word [3*4], int3entry
+	mov word [3*4+2], cs
+
+	pop ax
+	pop ds
+	retf
+
+crlf:	xor bx, bx
+	mov ax, 0x0e0a
+	int 0x10
+	mov al, 0x0d
+	int 0x10
+	ret
+
+printr: mov al, cl
+	int 0x10
+	mov al, ch
+	int 0x10
+	mov al, '='
+	int 0x10
+	call printd
+	mov al, ' '
+	int 0x10
+	ret
+
+printd:	mov cl, 4
+	; this double-call is essentially a 4 times repeating loop
+	call .c1
+.c1:	call .c2
+.c2:	; grab highest nibble from dx
+	mov al, dh
+	; remove highest nibble from dx
+	shl dx, cl
+	; shift away second-highest nibble that we accidentally copied
+	shr al, cl
+	; map 0-9 to ascii codes for '0' to '9'
+	add al, 0x30
+	; if result is larger than '9', ...
+	cmp al, 0x3a
+	jl .noadj
+	; ... add 7 so we continue at 'A'
+	add al, 7
+.noadj:	int 0x10
+	ret
+
+int3entry:
+	push bx
+	push dx
+	push cx
+	push ax
+	push bp
+	mov bp, sp
+
+	call crlf
+
+	xor bx, bx
+	mov ah, 0x0e
+	mov cx, 'AX'
+	mov dx, [bp+2]
+	call printr
+
+	mov cx, 'CX'
+	mov dx, [bp+4]
+	call printr
+
+	mov cx, 'DX'
+	mov dx, [bp+6]
+	call printr
+
+	mov cx, 'BX'
+	mov dx, [bp+8]
+	call printr
+
+	mov cx, 'BP'
+	mov dx, [bp]
+	call printr
+
+	mov cx, 'SP'
+	lea dx, [bp+16]
+	call printr
+
+	mov cx, 'SI'
+	mov dx, si
+	call printr
+
+	mov cx, 'DI'
+	mov dx, di
+	call printr
+
+	call crlf
+
+	mov cx, 'ES'
+	mov dx, es
+	call printr
+
+	mov cx, 'CS'
+	mov dx, [bp+12]
+	call printr
+
+	mov cx, 'SS'
+	mov dx, ss
+	call printr
+
+	mov cx, 'DS'
+	mov dx, ds
+	call printr
+
+	mov cx, 'IP'
+	mov dx, [bp+10]
+	call printr
+
+	call crlf
+
+	pop bp
+	pop ax
+	pop cx
+	pop dx
+	pop bx
+	iret
+
+align 512