Yay! It boots from hard disk, it boots from floppy, and it has a tolerable

(although not particularly helpful) error message, and no need to push more
code into LDLINUX.SYS.

diff --git a/ldlinux.asm b/ldlinux.asm
index 3e325c0..bc0c78a 100644 (file)
--- a/ldlinux.asm
+++ b/ldlinux.asm
@@ -401,10 +401,8 @@ floppy_table       equ $                   ; No sense in wasting memory, overwrite start
 
 start:
                cli                     ; No interrupts yet, please
-               jmp 0:start1            ; Some stupid BIOSes jumps to 07C0:0000 not 0000:7C00
-start1:
 ;
-; Set up ES and SS, and the stack
+; Set up the stack
 ;
                xor ax,ax
                mov ss,ax
@@ -424,8 +422,10 @@ start1:
 ; had to waste precious boot sector space with this code.
 ;
 ; This code no longer fits.  Hope that noone really needs it anymore.
+; In fact, some indications is that this code does more harm than good
+; with all the new kinds of drives and media.
 ;
-%ifdef SUPPORT_BROKEN_BIOSES
+%ifdef SUPPORT_REALLY_BROKEN_BIOSES
                lds si,[ss:fdctab]      ; DS:SI -> original
                push ds                 ; Save on stack in case
                push si                 ; we have to bail
@@ -449,11 +449,6 @@ start1:
 ;
                sti
 ;
-; Reset floppy system
-;
-               xor dx,dx
-               int 13h                 ; AH = 00h already
-;
 ; The drive number and possibly partition information was passed to us
 ; by the BIOS or previous boot loader (MBR).  Current "best practice" is to
 ; trust that rather than what the superblock contains.
@@ -493,16 +488,15 @@ not_harddisk:
 ; wasting precious boot sector space again...
 ;
 debugentrypt:
-               xor si,si               ; Some BIOSes destroy ES for INT 13:08
-               mov es,si               ; so we clear it this late
-
+               xor ax,ax               ; INT 13:08 destroys ES
+               mov es,ax               ; so we clear it this late
                mov al,[bsFATs]         ; Number of FATs (AH == 0)
                jc kaboom               ; If the floppy init failed
                mul word [bsFATsecs]    ; Get the size of the FAT area
                add ax,[bsHidden1]      ; Add hidden sectors
                adc dx,[bsHidden2]
                add ax,[bsResSectors]   ; And reserved sectors
-               adc dx,si               ; SI == 0
+               adc dx,byte 0
 
                mov [RootDir1],ax       ; Location of root directory
                mov [RootDir2],dx
@@ -523,7 +517,7 @@ debugentrypt:
                mov [EndofDirSec],bx    ; End of a single directory sector
 
                add [DataArea1],ax
-               adc word [DataArea2],si ; SI == 0
+               adc word [DataArea2],byte 0
 
                pop dx                  ; Reload root directory starting point
                pop ax
@@ -566,12 +560,11 @@ kaboom:
                pop word [si]           ; Restore location
                pop word [si+2]
                sti
-;              mov si,bailmsg
-;              call writestr           ; Returns with AL = 0
-;              cbw                     ; AH <- 0
-;              int 16h                 ; Wait for keypress
-;              int 19h                 ; And try once more to boot...
-               int 18h                 ; Bail.  Hope this does something useful.
+               mov si,bailmsg
+               call writestr           ; Returns with AL = 0
+               cbw                     ; AH <- 0
+               int 16h                 ; Wait for keypress
+               int 19h                 ; And try once more to boot...
 norge:         jmp short norge         ; If int 19h returned; this is the end
 
 ;
@@ -580,12 +573,12 @@ norge:            jmp short norge         ; If int 19h returned; this is the end
 ;
 found_it:      ; Note: we actually leave two words on the stack here
                ; (who cares?)
+               xor ax,ax
                mov al,[bsSecPerClust]
-               xor ah,ah
                mov bp,ax               ; Load an entire cluster
                mov bx,[si+26]          ; First cluster
                mov [RunLinClust],bx    ; Save for later use
-               dec bx                  ; First cluster is cluster 2
+               dec bx                  ; First cluster is "cluster 2"
                dec bx
                mul bx
                add ax,[DataArea1]
@@ -598,7 +591,12 @@ found_it:  ; Note: we actually leave two words on the stack here
                mov cx,magic_len
                repe cmpsb              ; Make sure that the bootsector
                jne kaboom              ; matches LDLINUX.SYS
-               jmp ldlinux_ent
+;
+; Some BIOSes are buggy and don't jump to 0000:7C00 but to an alias address.
+; We depend on CS == 0 later in the program, but in order to conserve space
+; we don't do this until here.
+;
+               jmp 0:ldlinux_ent
 ;
 ; writestr: write a null-terminated string to the console
 ;
@@ -610,7 +608,6 @@ wstr_1:         lodsb
                mov bx,0007h            ; White on black, current page
                int 10h
                jmp short wstr_1
-
 ;
 ; disk_error: decrement the retry count and bail if zero
 ;
@@ -621,6 +618,9 @@ disk_error: dec si                  ; SI holds the disk retry counter
                pop cx                  ; <H>
                pop dx                  ; <G>
                jmp short disk_try_again
+
+return:                ret
+
 ;
 ; getonesec: like getlinsec, but pre-sets the count to 1
 ;
@@ -726,9 +726,8 @@ disk_try_again: push dx                     ; <G>
                xor dx,dx               ; First head on new cylinder
 gls_nonewcyl:  sub cx,si               ; First sector on new track
                jmp short gls_nextchunk
-return:                ret
 
-; bailmsg              db 'Error!', 0Dh, 0Ah, 0
+bailmsg                db 'Boot failed!', 0Dh, 0Ah, 0
 
 bs_checkpt     equ $                   ; Must be <= 1E3h