use null-terminated strings for vga_println

boot1.s

@@ -214,7 +214,6 @@ main:
   call vga_clear
 
   mov ax, msg_boot1_loaded
-  mov cx, MSG_BOOT1_LOADED_LEN
   call vga_println
 
   call test_a20
@@ -308,12 +307,13 @@ vga_scroll:
   fnret
 
 
-; Write one line to the VGA text buffer.
+; 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:
-; - ax: pointer to the string to print
-; - cx: string length
+; - ax: pointer to the string to print, null-terminated
 ; Return: none
-; Clobber: ax, cx
+; Clobber: ax
 vga_println:
   fnstart
   push si
@@ -327,25 +327,26 @@ vga_println:
 .scroll_done:
 
   mov si, ax
-  mov ax, VGA_WIDTH
-  cmp cx, ax
-  cmova cx, ax
-  mov di, VGA_WIDTH * (VGA_HEIGHT - 1) * 2
 
   xor dx, dx
   mov ax, [GLOBALS + TEXTBUF_LINE]
   mov di, VGA_WIDTH * 2
   mul di
   mov di, ax
-  
+
   mov ax, 0xb800
   mov es, ax
   mov ah, [GLOBALS + VGA_COL + 1]
 
+  mov dx, VGA_WIDTH
+
 .loop:
-  or cx, cx
+  test dx, dx
   jz .done
+  dec dx
   mov al, [si]
+  test al, al
+  jz .done
   mov es:[di], ax
   add di, 2
   inc si
@@ -404,14 +405,10 @@ test_a20:
   ret
 
 
-msg_boot1_loaded     db  "boot1 loaded. hello!"
-MSG_BOOT1_LOADED_LEN equ $ - msg_boot1_loaded
-msg_a20_enabled      db  "a20 enabled"
-MSG_A20_ENABLED_LEN  equ $ - msg_a20_enabled
-msg_a20_disabled     db  "a20 disabled"
-MSG_A20_DISABLED_LEN equ $ - msg_a20_disabled
-msg_panic            db  "fuck"
-MSG_PANIC_LEN        equ $ - msg_panic
+msg_boot1_loaded db "boot1 loaded. hello!", 0
+msg_a20_enabled  db "a20 enabled", 0
+msg_a20_disabled db "a20 disabled", 0
+msg_panic        db "panic!", 0
 
 boot1_magic dd BOOT1_MAGIC