start work on panic handler

boot1.s

@@ -216,9 +216,9 @@ main:
   mov ax, msg_boot1_loaded
   call vga_println
 
-  mov ecx, 0xa1b2c3d4
-  call vga_dump_reg
-  hlt
+  mov eax, 0xa1b2c3d4
+  mov ebx, 0x12345678
+  call panic_fancy
 
   call test_a20
   test ax, ax
@@ -252,14 +252,50 @@ panic_default:
 
 ; Print a panic message then terminate.
 ; Arguments:
-; - [sp - 2]: pointer to panic message
-; - [sp]:     panic message length
+; - [sp - 2]: panic message segment
+; - [sp]:     panic message offset
 ; Does not return
 panic_fancy:
   push bp
   mov bp, sp
+
+  sub sp, 16 ; Buffer: bp - 16
+
+  push eax   ; bp - 20
+  push ebx   ; bp - 24
+  push ecx   ; bp - 28
+  push edx   ; bp - 32
+  push esi   ; bp - 36
+  push edi   ; bp - 40
+
   mov word [GLOBALS + VGA_COL], 0x4f00
+  call vga_clear
+
+  mov ax, ss
+  mov es, ax
+  
+  lea ax, [bp - 16]
+  push ax
+  mov ecx, [bp - 20]
+  call dump_reg
+  pop cx
+  xor ax, ax
+  mov dx, 8
+  call vga_print_raw
+
+  lea ax, [bp - 16]
+  push ax
+  mov ecx, [bp - 24]
+  call dump_reg
+  pop cx
+  mov ax, VGA_WIDTH
+  mov dx, 8
+  call vga_print_raw
+
+.halt:
   hlt
+  ; Handle non-maskable interrupts
+  jmp .halt
 
 
 ; Clear the VGA text buffer.
@@ -334,40 +370,42 @@ vga_scroll:
   fnret
 
 
-; Write one line to the VGA text buffer. The string should be null-terminated; we embrace the evil
-; of null-termination so this function only takes one argument, so it's slightly less of a faff to
-; call in most cases.
+; Write a null-terminated string to the given position in the VGA text buffer.
 ; Arguments:
 ; - es: output string segment
-; - ax: output string offset
-; Return: none
-; Clobber: ax
-vga_println:
+; - ax: vga buffer index
+; - cx: output string offset
+; - dx: maximum length of string to print
+; Return:
+; - ax: vga buffer index after last character written
+; Clobber: none
+vga_print_raw:
   fnstart
+  push fs
   push si
   push di
+  push cx
   push dx
-  push fs
-
-  cmp word [GLOBALS + TEXTBUF_LINE], VGA_HEIGHT
-  jb .scroll_done
-  call vga_scroll
-.scroll_done:
-
-  mov si, ax
-
-  xor dx, dx
-  mov ax, [GLOBALS + TEXTBUF_LINE]
-  mov di, VGA_WIDTH * 2
-  mul di
-  mov di, ax
 
+  mov si, cx
+  xchg ax, cx
+  
+  ; Find the distance between the starting index and the end of the buffer.
+  mov ax, (VGA_WIDTH * VGA_HEIGHT)
+  sub ax, cx
+  ; If the starting index is past the end of the buffer, return early.
+  jc .done
+  ; Clamp the maximum length to the distance between the starting index and the end of the buffer.
+  cmp ax, dx
+  cmovb dx, ax
+
+  mov di, cx
+  shl di, 1
+  
   mov ax, 0xb800
   mov fs, ax
   mov ah, [GLOBALS + VGA_COL + 1]
 
-  mov dx, VGA_WIDTH
-
 .loop:
   test dx, dx
   jz .done
@@ -378,38 +416,58 @@ vga_println:
   mov fs:[di], ax
   add di, 2
   inc si
-  dec cx
+  inc cx
   jmp .loop
-  
+
 .done:
-  inc word [GLOBALS + TEXTBUF_LINE]
-  
-  pop fs
+  xchg ax, cx
+
   pop dx
+  pop cx
   pop di
   pop si
+  pop fs
   fnret
 
 
-vga_dump_reg:
+; Write one line to the VGA text buffer. The string should be null-terminated; we embrace the evil
+; of null-termination so this function only takes one argument, so it's slightly less of a faff to
+; call in most cases.
+; Arguments:
+; - es: output string segment
+; - ax: output string offset
+; Return: none
+; Clobber: none
+vga_println:
   fnstart
-  sub sp, VGA_WIDTH
-  push es
+  push ax
   push bx
+  push cx
+  push dx
+
+  cmp word [GLOBALS + TEXTBUF_LINE], VGA_HEIGHT
+  jb .scroll_done
+  call vga_scroll
+.scroll_done:
 
-  mov ax, ss
-  mov es, ax
-  lea ax, [bp - VGA_WIDTH]
-  call dump_reg
   mov bx, ax
-  mov byte es:[bx], 0
+  
+  xor dx, dx
+  mov ax, [GLOBALS + TEXTBUF_LINE]
+  mov cx, VGA_WIDTH
+  mul cx
 
-  lea ax, [bp - VGA_WIDTH]
-  call vga_println
+  mov dx, VGA_WIDTH
+  mov cx, bx
+
+  call vga_print_raw
+
+  inc word [GLOBALS + TEXTBUF_LINE]
   
-  pop bx  
-  pop es
-  add sp, VGA_WIDTH
+  pop dx
+  pop cx
+  pop bx
+  pop ax
   fnret