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fat32 bs: use BPB include header and base off BP 

We now set BP once at the start to point to the beginning of the BPB.
Encoding an direct address access can then be done relative to BP, which
saves an immediate byte per access.
This commit is contained in:
Nero 2020-03-31 20:03:05 +00:00
parent 9044e3948c
commit 1981a8efba
1 changed files with 22 additions and 39 deletions
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61
boot/fat32.asm
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@ -4,29 +4,12 @@
jmp short init jmp short init
nop nop
; WORD reserved sector count %include "inc/bpb.asm"
rsc: equ ( $$ + 0x00E )
; BYTE number of sectors per cluster bpb: equ 0x080B
sc: equ ( $$ + 0x00D )
; BYTE number of FATs
fn: equ ( $$ + 0x010 )
; WORD number of root directory entries
rde: equ ( $$ + 0x011 )
; DWORD hidden sector count (partition offset)
po: equ ( $$ + 0x01C )
; DWORD sectors per FAT
sf: equ ( $$ + 0x024 )
; BYTE drive number (we set it from dl)
dn: equ ( $$ + 0x040 )
; Area for BPB ; Area for BPB
times (0x5A - ($-$$)) db 0 times ( 0x5A - ($-$$) ) db 0
init: xor ax, ax init: xor ax, ax
mov ss, ax mov ss, ax
@ -66,7 +49,7 @@ hlt: hlt
; offsets relative to FS ; offsets relative to FS
readd: ; add offset of cluster data area to DX:AX readd: ; add offset of cluster data area to DX:AX
push cx push cx
mov di, [rde] mov di, [bp+bpb_rde]
mov cl, 4 mov cl, 4
shr di, cl ; 32 bytes per entry shr di, cl ; 32 bytes per entry
add ax, di add ax, di
@ -75,17 +58,17 @@ readd: ; add offset of cluster data area to DX:AX
readr: ; add offset to rootdir to DX:AX (FAT12/FAT16 only) readr: ; add offset to rootdir to DX:AX (FAT12/FAT16 only)
push cx push cx
xor ch, ch xor ch, ch
mov cl, byte [fn] mov cl, byte [bp+bpb_fn]
fatlp: add ax, [sf] fatlp: add ax, [bp+bpb7_lsf]
adc dx, [sf+2] adc dx, [bp+bpb7_lsf+2]
loop fatlp loop fatlp
pop cx pop cx
readf: ; add offset to FAT table to DX:AX readf: ; add offset to FAT table to DX:AX
add ax, word [rsc] add ax, word [bp+bpb_rsc]
adc dx, 0 adc dx, 0
readp: ; read sector DX:AX from partition readp: ; read sector DX:AX from partition
add ax, word [po] add ax, word [bp+bpb_po]
adc dx, word [po+2] adc dx, word [bp+bpb_po+2]
jc err jc err
read_: ; read sector DX:AX from disk read_: ; read sector DX:AX from disk
; qword sector number DX:AX ; qword sector number DX:AX
@ -102,17 +85,17 @@ read_: ; read sector DX:AX from disk
push cx push cx
; size & passing ; size & passing
mov bp, 0x10 mov di, 0x10
push bp push di
mov si, sp mov si, sp
mov ah, 0x42 mov ah, 0x42
mov dl, [dn] mov dl, [bp+bpb7_dn]
stc stc
int 0x13 int 0x13
jc dskerr jc dskerr
add sp, bp add sp, di
ret ret
@ -159,7 +142,7 @@ readc: ; load cluster number
; convert cluster number to sector number ; convert cluster number to sector number
; this is some cheapo multiplication with 2^n ; this is some cheapo multiplication with 2^n
mov cl, [sc] mov cl, [bp+bpb_sc]
l02: shr cl, 1 l02: shr cl, 1
jz l02e jz l02e
clc clc
@ -168,15 +151,15 @@ l02: shr cl, 1
jmp l02 jmp l02
l02e: l02e:
; dx:ax is now sector num in data area ; dx:ax is now sector num in data area
mov cx, [sc] mov cx, [bp+bpb_sc]
xor ch, ch xor ch, ch
call readd call readd
xchg bp, cx xchg di, cx
mov cl, 5 mov cl, 5
sal bp, cl sal di, cl
add bx, bp add bx, di
pop dx pop dx
pop ax pop ax
@ -219,7 +202,8 @@ loadf: call search
mov dx, [si+0x14] mov dx, [si+0x14]
jmp load jmp load
main: mov [dn], dl main: mov bp, bpb
mov [bp+bpb7_dn], dl
; load root directory ; load root directory
call loadr call loadr
; search for first system directory ; search for first system directory
@ -234,10 +218,9 @@ main: mov [dn], dl
xor dx, dx xor dx, dx
xor bx, bx xor bx, bx
xor di, di xor di, di
xor bp, bp
; restore drive number ; restore drive number
mov dl, [dn] mov dl, [bp+bpb7_dn]
; restore potential partition table ; restore potential partition table
pop si pop si