vbr: implement file searching in rootdir

boot/vbr.asm

@@ -1,4 +1,4 @@
-org 0
+org 0x0500
 
 fdc:		; FDC Descriptor as per ECMA-107
   jmp _startup
@@ -47,23 +47,21 @@ _startup:
   xor ax, ax
   mov ds, ax
   mov es, ax
+  mov ss, ax
+  xor sp, sp
   mov cx, 0x0100
   mov si, 0x7C00
   mov di, 0x0500
-  rep movsw
-  jmp 0x0050:.lowstart
-.lowstart:
-  mov ax, cs
-  mov ds, ax
-  mov ss, ax
-  xor sp, sp
+  rep movsw ; relocate
+  jmp 0x0000:main
 
+main:
   mov [fdc.drv], dl ; backup drive number
 
-chs_bios:
+adjustchs:
   mov ah, 0x08
   int 0x13 ; required, QEMU detects 360kB as 1200kB with CHS 80 2 15
-  jc chs_print ; skip if function does not exist
+  jc loaddir ; skip if function does not exist
   inc dh
   mov [fdc.nos], dh
   mov ax, cx
@@ -77,72 +75,54 @@ chs_bios:
   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
-  jmp chs_print
 
-chs_print:
-  call print_inline
-  db "CHS ", 0
-
-  mov ax, [fdc.ts]
-  div word [fdc.nos]
-  div word [fdc.spt]
-  call print_number
-
-  call print_inline
-  db ",", 0
-
-  mov ax, [fdc.nos]
-  call print_number
-
-  call print_inline
-  db ",", 0
-
-  mov ax, [fdc.spt]
-  call print_number
-
-  call print_inline
-  db 0x0A, 0x0D, 0
-
-  mov ax, 0x0070
-  mov es, ax
-  xor bx, bx
-  call loadrootdir
-
-  call dump
-
-.end:
-  hlt
-  jmp .end
-
-; Load root directory into memory
-; in   es      data segment
-loadrootdir:
-  push bx
-  push cx
+loaddir:
   mov bx, [fdc.ss]	; bytes per sector
   mov cl, 5
   shr bx, cl		; div by 2^5, 32 bytes per directory entity
   mov ax, [fdc.rde]	; number of root directory entities
   xor dx, dx
   div bx
-  mov cx, ax
-
-  mov ax, [fdc.sf]
-  mul byte [fdc.fn]
-  add ax, [fdc.rsc]
-
-  xor bx, bx ; load at beginning of es
+  mov cx, ax		; cx = number of rootdir sectors
+  mov ax, [fdc.sf]      ; sectors per fat
+  mul byte [fdc.fn]     ; number of fats
+  add ax, [fdc.rsc]     ; ax = starting sector of rootdir
+  mov bx, buffer
 .loop:
   call loadblk
+  add bx, buffer
+  add ax, 1
   loop .loop
 
+scandir:
+  mov cx, [fdc.rde]
+  mov bx, buffer
+.loop:
+  push cx
+  mov cx, 0x000B
+  mov si, bx
+  mov di, bootfilename
+  repe cmpsb
   pop cx
-  pop bx
-  ret
+  jz found
+  add bx, 0x0020 ; 32 bytes per directory entry
+  loop .loop
+  call print_inline
+  db "File not found", 0x0A, 0x0D, 0
+  jmp hlt
+found:
+  add bx, 26
+  mov ax, [bx]
+  call print16
+  jmp hlt
+
+hlt:
+  hlt
+  jmp hlt
 
 ; Load a single block into memory
 ; in    ax      sector number
-;       es:bx	buffer
+;       bx	buffer
 ; out   al      error code,
 ;       carry set if error
 loadblk:
@@ -165,79 +145,41 @@ loadblk:
   pop dx
   pop cx
   pop ax
-  inc ax
-  add bx, 0x0200
   ret
 .error:
-  xor al, al
   xchg al, ah
   call print_inline
   db "disk error ", 0
-  call print_number
+  call print8
   call print_inline
   db 0x0A, 0x0D, 0
-.hlt:
-  hlt
-  jmp .hlt
-
-print_number:
-  mov cx, 0x000A
-  xor dx, dx
-  div cx ; ax = dx:ax / 10, dx = dx:ax % 10
-  and ax, ax
-  jz .print
-  push dx
-  call print_number
-  pop dx
-.print:
-  mov al, dl
-  add al, 0x30
-  mov ah, 0x0e
-  int 0x10
-  ret
-
-print_inline:
-  pop si
-  push ax
-  push bx
-.loop:
-  lodsb
-  cmp al, 0x00
-  je .end
-  mov bx, 0x0000
-  mov ah, 0x0e
-  int 0x10
-  cmp al, 0x0D
-  jmp .loop
-.end:
-  pop bx
-  pop ax
-  push si
-  ret
+  jmp hlt
 
+; in	si
 dump:
   mov si, bx
   mov cx, 0x10
 .nextline:
   mov ax, si
   call print16
-  mov al, 0x3A
-  call printc
-  mov al, 0x20
-  call printc
+
+  push si
+  call print_inline
+  db 0x3A, 0x20, 0
+  pop si
 
   push cx
   mov cx, 0x0020
-.words:
-  es lodsb
+.bytes:
+  lodsb
   call print8
-  loop .words
+  loop .bytes
   pop cx
 
-  mov al, 0x0A
-  call printc
-  mov al, 0x0D
-  call printc
+  push si
+  call print_inline
+  db 0x0A, 0x0D, 0
+  pop si
 
   loop .nextline
 
@@ -247,7 +189,7 @@ dump:
 
 times (0x1F0 - ($-$$)) db 0
 
-str_bootfile:
+bootfilename:
   db "BOOT"
   times (0x1F8 - ($-$$)) db " "
   db "BIN"
@@ -255,3 +197,5 @@ str_bootfile:
 times (0x1FE - ($-$$)) db 0
 
 dw 0xAA55
+
+buffer:

lib/print.asm

@@ -0,0 +1,66 @@
+; important functions in this file: kprintf
+
+; write a character to kernel output
+; in: al
+printc:
+  push ax
+  push bx
+  mov bx, 0x0000
+  mov ah, 0x0e
+  int 0x10
+  pop bx
+  pop ax
+  ret
+
+; prints a nibble in hex
+; in: al
+print4:
+  and al, 0x0F
+  add al, 0x30
+  cmp al, 0x3a
+  jl printc
+  add al, 0x07
+  jmp printc
+
+; print a byte
+; in: al
+print8:
+  push ax ; avoid destroying ah
+  aam 16 ; high nibble moved into ah
+  xchg ah,al ; high nibble first
+  call print4
+  xchg ah,al
+  call print4
+  pop ax
+  ret
+
+; print a word
+; in: ax
+print16:
+  xchg ah,al
+  call print8
+  xchg ah,al
+  call print8
+  ret
+
+; print a inline following its call opcode
+; the string ends with a nullbyte
+; trashes si
+print_inline:
+  pop si
+  push ax
+  push bx
+  mov bx, 0x0000
+.loop:
+  lodsb
+  cmp al, 0x00
+  je .end
+  mov ah, 0x0e
+  int 0x10
+  cmp al, 0x0D
+  jmp .loop
+.end:
+  pop bx
+  pop ax
+  push si
+  ret