function for dumping register value

boot1.s

@@ -216,25 +216,52 @@ main:
   mov ax, msg_boot1_loaded
   call vga_println
 
+  mov ecx, 0xa1b2c3d4
+  call vga_dump_reg
+  hlt
+
   call test_a20
   test ax, ax
   jnz .a20_enabled
   
   mov ax, msg_a20_disabled
-  mov cx, MSG_A20_DISABLED_LEN
   call vga_println
 
   ; TODO: enable a20
   hlt
 
-.a20_enabled
+.a20_enabled:
   mov ax, msg_a20_enabled
-  mov cx, MSG_A20_ENABLED_LEN
   call vga_println
   
   hlt
 
 
+; Wrapper function around panic_fancy which provides default arguments.
+; Arguments: none
+; Does not return
+panic_default:
+  push bp
+  mov bp, sp
+  push word 0
+  push word 0
+  ; Spoof return address
+  push dword [bp]
+  jmp panic_fancy
+
+
+; Print a panic message then terminate.
+; Arguments:
+; - [sp - 2]: pointer to panic message
+; - [sp]:     panic message length
+; Does not return
+panic_fancy:
+  push bp
+  mov bp, sp
+  mov word [GLOBALS + VGA_COL], 0x4f00
+  hlt
+
+
 ; Clear the VGA text buffer.
 ; Arguments: none
 ; Return: none
@@ -311,7 +338,8 @@ vga_scroll:
 ; of null-termination so this function only takes one argument, so it's slightly less of a faff to
 ; call in most cases.
 ; Arguments:
-; - ax: pointer to the string to print, null-terminated
+; - es: output string segment
+; - ax: output string offset
 ; Return: none
 ; Clobber: ax
 vga_println:
@@ -319,7 +347,7 @@ vga_println:
   push si
   push di
   push dx
-  push es
+  push fs
 
   cmp word [GLOBALS + TEXTBUF_LINE], VGA_HEIGHT
   jb .scroll_done
@@ -335,7 +363,7 @@ vga_println:
   mov di, ax
 
   mov ax, 0xb800
-  mov es, ax
+  mov fs, ax
   mov ah, [GLOBALS + VGA_COL + 1]
 
   mov dx, VGA_WIDTH
@@ -344,10 +372,10 @@ vga_println:
   test dx, dx
   jz .done
   dec dx
-  mov al, [si]
+  mov al, es:[si]
   test al, al
   jz .done
-  mov es:[di], ax
+  mov fs:[di], ax
   add di, 2
   inc si
   dec cx
@@ -356,13 +384,100 @@ vga_println:
 .done:
   inc word [GLOBALS + TEXTBUF_LINE]
   
-  pop es
+  pop fs
   pop dx
   pop di
   pop si
   fnret
 
 
+vga_dump_reg:
+  fnstart
+  sub sp, VGA_WIDTH
+  push es
+  push bx
+
+  mov ax, ss
+  mov es, ax
+  lea ax, [bp - VGA_WIDTH]
+  call dump_reg
+  mov bx, ax
+  mov byte es:[bx], 0
+
+  lea ax, [bp - VGA_WIDTH]
+  call vga_println
+  
+  pop bx  
+  pop es
+  add sp, VGA_WIDTH
+  fnret
+
+
+; Convert the value in ecx to hex and write it to the buffer at es:ax. The buffer should be at
+; least 8 bytes long.
+; - es: output buffer segment
+; - ax: output buffer offset
+; - ecx: value to convert to hex and print
+; Return:
+; - ax: the address one after the last byte that was written
+; Clobber: none
+dump_reg:
+  fnstart
+  push bx
+  push dx
+  push ecx
+  
+  mov bx, ax
+  mov dx, 4
+
+.loop:
+  test dx, dx
+  jz .done
+  dec dx
+
+  rol ecx, 8
+
+  mov al, cl
+  shr al, 4
+  call nybble_to_hex_char
+  mov es:[bx], al
+  inc bx
+  
+  mov al, cl
+  call nybble_to_hex_char
+  mov es:[bx], al
+  inc bx
+
+  jmp .loop
+
+.done:
+  mov ax, bx
+
+  pop ecx
+  pop dx
+  pop bx
+  fnret
+
+
+; Convert nybble to lowercase ascii hex char.
+; Arguments:
+; - al: value to convert
+; Return:
+; - al: converted ascii hex value
+; Clobber: none
+nybble_to_hex_char:
+  ; We don't use the stack, so no need to change bp.
+  and al, 0x0f
+  cmp al, 9
+  jbe .0_to_9
+  add al, (0x61 - 0x0a)
+  jmp .done
+.0_to_9:
+  add al, 0x30
+.done:
+  ret
+
+
 enable_a20:
   fnstart
   fnret
@@ -407,7 +522,7 @@ test_a20:
 
 msg_boot1_loaded db "boot1 loaded. hello!", 0
 msg_a20_enabled  db "a20 enabled", 0
-msg_a20_disabled db "a20 disabled", 0
+msg_a20_disabled db "a20 not enabled", 0
 msg_panic        db "panic!", 0
 
 boot1_magic dd BOOT1_MAGIC