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boot: fix Int10.0E expected not to alter al (SBC188)

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Reference: https://www.bttr-software.de/forum/forum_entry.php?id=16934

The SBC188 video ROM-BIOS handling returns with al=0Ah (LF)
upon interrupt 10h function 0Eh calls that scroll the screen.
This made the FAT12 boot loader crash when booting FreeDOS
off a 1440 KiB diskette. While a fix is expected from their
side too, hardening the loader too is the right thing to do.

With the way this fix is implemented by this commit, it only
needs one additional byte for the loader. The error message
changes from "Error!." to just "Error!", other than
that the behaviour is the same.

The FAT32 loaders already used zero-terminated strings and
thus were not affected by the dependence.
This commit is contained in:
C. Masloch 2020-09-15 15:45:31 +02:00 committed by Kenneth J Davis
parent ef945b1192
commit 977023f85c
2 changed files with 16 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
boot/boot.asm
View File

@ -355,18 +355,19 @@ load_next: dec ax ; cluster numbers start with 2
; shows text after the call to this function.
show.do_show:
mov ah, 0Eh ; show character
int 10h ; via "TTY" mode
show: pop si
lodsb ; get character
push si ; stack up potential return address
mov ah,0x0E ; show character
int 0x10 ; via "TTY" mode
cmp al,'.' ; end of string?
jne show ; until done
cmp al, 0 ; end of string?
jne .do_show ; until done
ret
boot_error: call show
; db "Error! Hit a key to reboot."
db "Error!."
; db "Error! Hit a key to reboot.",0
db "Error!",0
xor ah,ah
int 0x13 ; reset floppy
@ -391,7 +392,7 @@ readDisk: push si
mov word [READADDR_OFF], bx
call show
db "."
db ".",0
read_next:
;******************** LBA_READ *******************************

15
boot/oemboot.asm
View File

@ -460,8 +460,8 @@ cluster_next: lodsw ; AX = next cluster to read
; failed to boot
boot_error:
call show
; db "Error! Hit a key to reboot."
db "):."
; db "Error! Hit a key to reboot.",0
db "):",0
%ifdef LOOPONERR
jmp $
%else
@ -488,13 +488,14 @@ load_next: dec ax ; cluster numbers start with 2
; shows text after the call to this function.
show.do_show:
mov ah, 0Eh ; show character
int 10h ; via "TTY" mode
show: pop si
lodsb ; get character
push si ; stack up potential return address
mov ah,0x0E ; show character
int 0x10 ; via "TTY" mode
cmp al,'.' ; end of string?
jne show ; until done
cmp al, 0 ; end of string?
jne .do_show ; until done
ret
@ -516,7 +517,7 @@ readDisk: push si ; preserve cluster #
mov word [LBA_OFF], bx
call show
db "."
db ".",0
read_next:
; initialize constants