From 977023f85c701d740260ccffea373d333cce676e Mon Sep 17 00:00:00 2001 From: "C. Masloch" Date: Tue, 15 Sep 2020 15:45:31 +0200 Subject: [PATCH] boot: fix Int10.0E expected not to alter al (SBC188) 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. --- boot/boot.asm | 15 ++++++++------- boot/oemboot.asm | 15 ++++++++------- 2 files changed, 16 insertions(+), 14 deletions(-) diff --git a/boot/boot.asm b/boot/boot.asm index a7c88a6..d1ae0f5 100644 --- a/boot/boot.asm +++ b/boot/boot.asm @@ -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 ******************************* diff --git a/boot/oemboot.asm b/boot/oemboot.asm index a57bdbc..fa16ab6 100644 --- a/boot/oemboot.asm +++ b/boot/oemboot.asm @@ -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