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fat32 bs: use BPB include header and base off BP

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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
nop
; WORD reserved sector count
rsc: equ ( $$ + 0x00E )
%include "inc/bpb.asm"
; BYTE number of sectors per cluster
sc: equ ( $$ + 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 ( $$ + 0x040 )
bpb: equ 0x080B
; Area for BPB
times (0x5A - ($-$$)) db 0
times ( 0x5A - ($-$$) ) db 0
init: xor ax, ax
mov ss, ax
@ -66,7 +49,7 @@ hlt: hlt
; offsets relative to FS
readd: ; add offset of cluster data area to DX:AX
push cx
mov di, [rde]
mov di, [bp+bpb_rde]
mov cl, 4
shr di, cl ; 32 bytes per entry
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)
push cx
xor ch, ch
mov cl, byte [fn]
fatlp: add ax, [sf]
adc dx, [sf+2]
mov cl, byte [bp+bpb_fn]
fatlp: add ax, [bp+bpb7_lsf]
adc dx, [bp+bpb7_lsf+2]
loop fatlp
pop cx
readf: ; add offset to FAT table to DX:AX
add ax, word [rsc]
add ax, word [bp+bpb_rsc]
adc dx, 0
readp: ; read sector DX:AX from partition
add ax, word [po]
adc dx, word [po+2]
add ax, word [bp+bpb_po]
adc dx, word [bp+bpb_po+2]
jc err
read_: ; read sector DX:AX from disk
; qword sector number DX:AX
@ -102,17 +85,17 @@ read_: ; read sector DX:AX from disk
push cx
; size & passing
mov bp, 0x10
push bp
mov di, 0x10
push di
mov si, sp
mov ah, 0x42
mov dl, [dn]
mov dl, [bp+bpb7_dn]
stc
int 0x13
jc dskerr
add sp, bp
add sp, di
ret
@ -159,7 +142,7 @@ readc: ; load cluster number
; convert cluster number to sector number
; this is some cheapo multiplication with 2^n
mov cl, [sc]
mov cl, [bp+bpb_sc]
l02: shr cl, 1
jz l02e
clc
@ -168,15 +151,15 @@ l02: shr cl, 1
jmp l02
l02e:
; dx:ax is now sector num in data area
mov cx, [sc]
mov cx, [bp+bpb_sc]
xor ch, ch
call readd
xchg bp, cx
xchg di, cx
mov cl, 5
sal bp, cl
add bx, bp
sal di, cl
add bx, di
pop dx
pop ax
@ -219,7 +202,8 @@ loadf: call search
mov dx, [si+0x14]
jmp load
main: mov [dn], dl
main: mov bp, bpb
mov [bp+bpb7_dn], dl
; load root directory
call loadr
; search for first system directory
@ -234,10 +218,9 @@ main: mov [dn], dl
xor dx, dx
xor bx, bx
xor di, di
xor bp, bp
; restore drive number
mov dl, [dn]
mov dl, [bp+bpb7_dn]
; restore potential partition table
pop si