vbr: Reorder code, improve error behavior

boot/fat12vbr.asm

@@ -1,4 +1,4 @@
-org 0x0500
+org 0x0600
 
 fdc:		; FDC Descriptor as per ECMA-107
   jmp _startup
@@ -51,7 +51,7 @@ _startup:
   xor sp, sp
   ; relocate
   mov si, 0x7C00
-  mov di, 0x0500
+  mov di, 0x0600
   mov cx, 0x0100
   rep movsw
   ; adjust CS
@@ -60,10 +60,35 @@ _startup:
 main:
   mov [fdc.drv], dl ; backup drive number
 
-adjustchs:
+  call fix_chs
+  call find_first_sector
+
+  call error
+  db "END OF IMPLEMENTATION", 0
+
+; Write AX, a string and bail out
+; Return pointer is the string start
+; String must be 0 suffixed
+error:
+  call print16
+  pop si
+  mov bx, 0x0000
+  mov al, 0x20
+.loop:
+  mov ah, 0x0e
+  int 0x10
+  lodsb
+  cmp al, 0x00
+  jne .loop
+.hlt:
+  hlt
+  jmp .hlt
+
+; Read CHS data from BIOS and fix fdc values
+fix_chs:
   mov ah, 0x08
   int 0x13
-  jc loaddir ; skip if function does not exist
+  jc .end ; skip if function does not exist
   inc dh
   mov [fdc.nos], dh
   mov ax, cx
@@ -77,8 +102,11 @@ adjustchs:
   mul word [fdc.nos] ; number of tracks = number of cylinders * heads
   mul word [fdc.spt] ; number of sectors = number of tracks * sectors per track
   mov [fdc.ts], ax
+.end:
+  ret
 
-loaddir:
+; Load the root directory [buffer]
+find_first_sector:
   mov bx, [fdc.ss]      ; bytes per sector
   mov cl, 5
   shr bx, cl            ; div by 2^5, 32 bytes per directory entity
@@ -90,56 +118,38 @@ loaddir:
   mul byte [fdc.fn]     ; number of fats
   add ax, [fdc.rsc]     ; ax = starting sector of rootdir
   mov bx, buffer
-.loop:
+.blkloop:
   call loadblk
   add bx, buffer
   add ax, 1
-  loop .loop
-
-scandir:
+  loop .blkloop
   mov cx, [fdc.rde]
   mov bx, buffer
-.loop:
+.dirloop:
   push cx
   mov cx, 0x000B
   mov si, bx
   mov di, bootfilename
   repe cmpsb
   pop cx
-  jz found
+  jz .found
   add bx, 0x0020 ; 32 bytes per directory entry
-  loop .loop
+  loop .dirloop
   call error
   db "NOT FOUND", 0
-found:
+.found:
   add bx, 26
   mov ax, [bx]
-  call print16
-  jmp hlt
-
-error:
-  call print16
-  pop si
-  mov bx, 0x0000
-  mov al, 0x20
-.loop:
-  mov ah, 0x0e
-  int 0x10
-  lodsb
-  cmp al, 0x00
-  jne .loop
-  ; fall through into htl
-
-hlt:
-  hlt
-  jmp hlt
+  ret
 
 ; Load a single FAT cluster into memory
 ; in    ax      cluster number
 ;       bx      buffer
 ;
 loadsec:
-
+  call print16
+  mov ax, bx
+  call print16
   ret
 
 ; Load a single block into memory