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kernel
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boot: enable loading from file systems with 256 sectors per cluster 

This should be Enhanced DR-DOS compatible.

oemboot.asm contains an optimisation to save 2 bytes,
by using mov instead of a lea with bp evaluating to
a constant value. This was needed to fit the addition
for 256 spc support. It is otherwise unrelated.

The LBA detection patch offsets in SYS have also been
updated to match the changed boot.asm offsets.
This commit is contained in:
C. Masloch 2023-01-04 22:09:46 +01:00 committed by Kenneth J Davis
parent 28cd58688a
commit 2efe4ab9f4
5 changed files with 18 additions and 10 deletions
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8
boot/boot.asm
View File

@ -363,7 +363,9 @@ load_next: dec ax ; cluster numbers start with 2
dec ax dec ax
mov di, word [bsSecPerClust] mov di, word [bsSecPerClust]
and di, 0xff ; DI = sectors per cluster dec di ; minus one if 256 spc
and di, 0xff ; DI = sectors per cluster - 1
inc di ; = spc
mul di mul di
add ax, [data_start] add ax, [data_start]
adc dx, [data_start+2] ; DX:AX = first sector to read adc dx, [data_start+2] ; DX:AX = first sector to read
@ -422,9 +424,9 @@ read_next:
; NOTE: sys must be updated if location changes!!! ; NOTE: sys must be updated if location changes!!!
%ifdef ISFAT12 %ifdef ISFAT12
%define LBA_TEST_OFFSET 179h %define LBA_TEST_OFFSET 17Bh
%elifdef ISFAT16 %elifdef ISFAT16
%define LBA_TEST_OFFSET 176h %define LBA_TEST_OFFSET 178h
%else %else
%define LBA_TEST_OFFSET 0 %define LBA_TEST_OFFSET 0
; Just a placeholder, so the proper error message ; Just a placeholder, so the proper error message

4
boot/boot32.asm
View File

@ -294,7 +294,9 @@ c3:
sub ax, 2 sub ax, 2
sbb cx, byte 0 ; CX:AX == cluster - 2 sbb cx, byte 0 ; CX:AX == cluster - 2
mov bl, [bsSecPerClust] mov bl, [bsSecPerClust]
sub bh, bh dec bx ; bl = spc - 1, 0FFh if 256 spc
sub bh, bh ; bx = spc - 1
inc bx ; bx = spc
xchg cx, ax ; AX:CX == cluster - 2 xchg cx, ax ; AX:CX == cluster - 2
mul bx ; first handle high word mul bx ; first handle high word
; DX must be 0 here ; DX must be 0 here

2
boot/boot32lb.asm
View File

@ -313,6 +313,8 @@ convert_cluster:
dec eax dec eax
movzx edx, byte [bsSecPerClust] movzx edx, byte [bsSecPerClust]
dec dl ; spc - 1
inc dx ; spc
push edx push edx
mul edx mul edx
pop edx pop edx

12
boot/oemboot.asm
View File

@ -143,7 +143,7 @@ segment .text
%define FATBUF bp-0x7500 ; offset of temporary buffer for FAT %define FATBUF bp-0x7500 ; offset of temporary buffer for FAT
; chain 0:FATBUF = 0:0700 = LOADSEG:0 ; chain 0:FATBUF = 0:0700 = LOADSEG:0
%define ROOTDIR bp-0x7700 ; offset to buffer for root directory %define ROOTDIR 0x7C00-0x7700 ; offset to buffer for root directory
; entry of kernel 0:ROOTDIR ; entry of kernel 0:ROOTDIR
%define CLUSTLIST bp+0x0300 ; zero terminated list of clusters %define CLUSTLIST bp+0x0300 ; zero terminated list of clusters
; that the kernel occupies ; that the kernel occupies
@ -332,11 +332,11 @@ real_start:
pop di ; mov di, word [RootDirSecs] pop di ; mov di, word [RootDirSecs]
pop ax ; mov ax, word [root_dir_start] pop ax ; mov ax, word [root_dir_start]
pop dx ; mov dx, word [root_dir_start+2] pop dx ; mov dx, word [root_dir_start+2]
lea bx, [ROOTDIR] ; es:bx = 0:0500 mov bx, ROOTDIR ; es:bx = 0:0500
push es ; save pointer to ROOTDIR push es ; save pointer to ROOTDIR
call readDisk call readDisk
pop es ; restore pointer to ROOTDIR pop es ; restore pointer to ROOTDIR
lea si, [ROOTDIR] ; ds:si = 0:0500 mov si, ROOTDIR ; ds:si = 0:0500
; Search for kernel file name, and find start cluster. ; Search for kernel file name, and find start cluster.
@ -361,7 +361,7 @@ ffDone:
%ifdef SETROOTDIR %ifdef SETROOTDIR
; copy over this portion of root dir to 0x0:500 for PC-DOS ; copy over this portion of root dir to 0x0:500 for PC-DOS
; (this may allow IBMBIO.COM to start in any directory entry) ; (this may allow IBMBIO.COM to start in any directory entry)
lea di, [ROOTDIR] ; es:di = 0:0500 mov di, ROOTDIR ; es:di = 0:0500
mov cx, 32 ; limit to this 1 entry (rest don't matter) mov cx, 32 ; limit to this 1 entry (rest don't matter)
rep movsw rep movsw
%endif %endif
@ -493,7 +493,9 @@ load_next: dec ax ; cluster numbers start with 2
dec ax dec ax
mov di, word [bsSecPerClust] mov di, word [bsSecPerClust]
and di, 0xff ; DI = sectors per cluster dec di ; minus one if 256 spc
and di, 0xff ; DI = sectors per cluster - 1
inc di ; = spc
mul di mul di
add ax, [data_start] add ax, [data_start]
adc dx, [data_start+2] ; DX:AX = first sector to read adc dx, [data_start+2] ; DX:AX = first sector to read

2
sys/sys.c
View File

@ -1567,7 +1567,7 @@ void put_boot(SYSOptions *opts)
/* !!! if boot sector changes then update these locations !!! */ /* !!! if boot sector changes then update these locations !!! */
{ {
unsigned offset; unsigned offset;
offset = (fs == FAT16) ? 0x176 : 0x179; offset = (fs == FAT16) ? 0x178 : 0x17B;
if ( (newboot[offset]==0x84) && (newboot[offset+1]==0xD2) ) /* test dl,dl */ if ( (newboot[offset]==0x84) && (newboot[offset+1]==0xD2) ) /* test dl,dl */
{ {