From c5afc6ec618461a7b1af24b372938b07e139f559 Mon Sep 17 00:00:00 2001
From: Nero <41307858+nero@users.noreply.github.com>
Date: Sat, 26 Dec 2020 16:03:15 +0000
Subject: [PATCH] Add printf facility for kernel to allow debug messages on
 phys. system

---
 boot/kernel.asm   | 14 +++-------
 kernel/drive.asm  | 20 +++++++++++---
 kernel/printf.asm | 66 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 87 insertions(+), 13 deletions(-)
 create mode 100644 kernel/printf.asm

diff --git a/boot/kernel.asm b/boot/kernel.asm
index e190edf..c806402 100644
--- a/boot/kernel.asm
+++ b/boot/kernel.asm
@@ -16,13 +16,9 @@ init:		cli
 		mov ss, ax
 		mov sp, ( stack+stacksize )
 
-print_banner:	mov si, banner
-		mov ah, 0x0e
-		xor bx, bx
-.loop:		lodsb
-		int 0x10
-		cmp al, 0x0D
-		jnz .loop
+		call printf
+%defstr VERSIONSTR VERSION
+		db "RDOS ", VERSIONSTR, 0x0A, 0x0D, 0
 
 		push dx
 		call dinit
@@ -50,12 +46,10 @@ hlt:		hlt
 %include	"kernel/far.asm"
 %include	"kernel/fcb.asm"
 %include        "kernel/drive.asm"
+%include	"kernel/printf.asm"
 
 section .data
 
-%defstr VERSIONSTR VERSION
-banner:		db "RDOS ", VERSIONSTR, 0x0A, 0x0D
-
 testfcb:	db 0
 		db "HELLO   ", "COM"
 		times 30 db 0
diff --git a/kernel/drive.asm b/kernel/drive.asm
index 7f9a0eb..d49f869 100644
--- a/kernel/drive.asm
+++ b/kernel/drive.asm
@@ -46,9 +46,21 @@ dinit:		; copy previously set DPT to our data area
 
 		; int 13 stub
 		; place reserved for stack switches
-int13:		;int 3
+int13:		; debug output
+		push bx
+		push dx
+		push cx
+		push ax
+		call printf
+		db "int13 CALL AX=",2," CX=",2," DX=",2," BX=",2,0x0A,0x0D,0
+		; do the call
 		int 0x13
-		;int 3
+		jc .err
+		ret
+.err:		push ax
+		call printf
+		db "int13 ERR AX=",2,0x0A,0x0D,0
+		stc
 		ret
 
 		; restore DPT to default values if possible
@@ -156,7 +168,9 @@ read:		push ax
 		mov [dskseek+2], dx
 		; do the actual read
 
-		; set the operation code and back it up to the stack
+		; low level read and write
+		; call again to retry, no input registers
+		; read or write is configured in cx
 _read:		mov ch, 2
 		db 0x3D ; cmp ax, imm16: causes next instr to be skipped
 _write:		mov ch, 3
diff --git a/kernel/printf.asm b/kernel/printf.asm
new file mode 100644
index 0000000..bda3f30
--- /dev/null
+++ b/kernel/printf.asm
@@ -0,0 +1,66 @@
+section .bss
+
+printr:
+.ax:	resw 1
+.cx:	resw 1
+.dx:	resw 1
+.bx:	resw 1
+
+section .text
+
+; Stack contents
+; ret-addr arg1 arg2 arg3 ...
+
+printf:		mov [printr.ax], ax
+		mov [printr.cx], cx
+		mov [printr.dx], dx
+		mov [printr.bx], bx
+		pop bx
+
+.loop:		mov al, [cs:bx]
+		inc bx
+
+		cmp al, 0
+		je .end
+		cmp al, 2
+		je .word
+
+		call pputc
+
+		jmp .loop
+
+.end:		push bx
+		mov ax, [printr.ax]
+		mov cx, [printr.cx]
+		mov dx, [printr.dx]
+		mov bx, [printr.bx]
+		ret
+
+.word:		pop dx
+		call pdx
+		jmp printf.loop
+
+pdx:		; this double-call is essentially a 4 times repeating loop
+		call .l1
+.l1:		call .l2
+.l2:		; set up cl for bit shifts
+		mov cl, 4
+		; 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 pputc
+		; ... add 7 so we continue at 'A'
+		add al, 7
+pputc:		push bx
+		mov ah, 0x0e
+		xor bx, bx
+		int 0x10
+		pop bx
+		ret