Be ultra-hyper-paranoid about A20.

diff --git a/NEWS b/NEWS
index 9b93284..4214d54 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,11 @@
 Changes in 1.42:
        * Frob the "fast A20 gate" port as well as the keyboard
          controller; will this help systems with problems?
+       * Be even more paranoid about A20, unfortunately even this
+         seems to be not paranoid enough... what I don't understand
+         is that if there is hardware out there *this broken*, how
+         can it run Linux at all?  Report an error message rather
+         than hang forever if A20 is stuck.
        * Include some intermediate files in the distribution, plus
          provide a "make installer" target for distributors to relink
          the install programs only.  I would prefer the syslinux boot

diff --git a/ldlinux.asm b/ldlinux.asm
index dcac21c..55ce99c 100644 (file)
--- a/ldlinux.asm
+++ b/ldlinux.asm
@@ -343,7 +343,7 @@ InitRDCNameLen  resw 1                      ; Length of unmangled initrd name
 NextCharJump    resw 1                 ; Routine to interpret next print char
 SetupSecs      resw 1                  ; Number of setup sectors
 SavedSP                resw 1                  ; Our SP while running a COMBOOT image
-A20test                resw 1                  ; Counter for testing status of A20
+A20Test                resw 1                  ; Counter for testing status of A20
 TextAttrBX      equ $
 TextAttribute   resb 1                 ; Text attribute for message file
 TextPage        resb 1                 ; Active display page
@@ -356,6 +356,7 @@ VidRows         resb 1                      ; Rows on screen-1
 RetryCount      resb 1                 ; Used for disk access retries
 KbdFlags       resb 1                  ; Check for keyboard escapes
 LoadFlags      resb 1                  ; Loadflags from kernel
+A20Tries       resb 1                  ; Times until giving up on A20
 MNameBuf        resb 11                ; Generic mangled file name buffer
 InitRD          resb 11                 ; initrd= mangled name
 KernelCName     resb 13                 ; Unmangled kernel name
@@ -2272,13 +2273,10 @@ cwstr_2:        popa
 ; segments, but this stuff is painful enough as it is without having to rely
 ; on everything happening "as it ought to."
 ;
-               align 2
-bcopy_gdt_ptr: dw bcopy_gdt_size-1
-               dd bcopy_gdt
-
                align 4
-bcopy_gdt:     dd 0                    ; Null descriptor
-               dd 0
+bcopy_gdt:     dw bcopy_gdt_size-1     ; Null descriptor - contains GDT
+               dd bcopy_gdt            ; pointer for LGDT instruction
+               dw 0
                dd 0000ffffh            ; Code segment, use16, readable,
                dd 00009b00h            ; present, dpl 0, cover 64K
                dd 0000ffffh            ; Data segment, use16, read/write,
@@ -2294,23 +2292,20 @@ bcopy:          push eax
                push ds
                push es
 
-               mov al,'<'              ; DEBUG DEBUG DEBUG
-               call writechr           
-
                cli
                call enable_a20
 
-               o32 lgdt [bcopy_gdt_ptr]
+               o32 lgdt [bcopy_gdt]
                mov eax,cr0
                or al,1
                mov cr0,eax             ; Enter protected mode
-               jmp 8:.in_pm
+               jmp 08h:.in_pm
 
-.in_pm:                mov ax,16               ; Data segment selector
+.in_pm:                mov ax,10h              ; Data segment selector
                mov es,ax
                mov ds,ax
 
-               mov al,24               ; "Real-mode-like" data segment
+               mov al,18h              ; "Real-mode-like" data segment
                mov ss,ax
                mov fs,ax
                mov gs,ax       
@@ -2321,7 +2316,7 @@ bcopy:            push eax
                mov ds,ax
        
                mov eax,cr0
-               and al,0feh
+               and al,~1
                mov cr0,eax             ; Disable protected mode
                jmp 0:.in_rm
 
@@ -2333,9 +2328,6 @@ bcopy:            push eax
                pop gs
                call disable_a20
 
-               mov al,'>'              ; DEBUG DEBUG DEBUG
-               call writechr           
-
                popf                    ; Re-enables interrupts
                pop eax
                ret
@@ -2350,9 +2342,13 @@ bcopy:           push eax
 ; We typically toggle A20 twice for every 64K transferred.
 ; 
 %define        io_delay  times 4 out 0EDh,al   ; Invalid port (we hope)
-%define delaytime 256                  ; 4 x ISA bus cycles (@ 1.5 µs)
+%define delaytime 1024                 ; 4 x ISA bus cycles (@ 1.5 µs)
+
 
 enable_a20:
+               mov byte [ss:A20Tries],255 ; Times to try to make this work
+
+try_enable_a20:
 ;
 ; Flush the caches
 ;
@@ -2361,9 +2357,9 @@ enable_a20:
 ;
 ; Enable the "fast A20 gate"
 ;
-               in al, 92h
+               in al, 092h
                or al,02h
-               out 92h, al
+               out 092h, al
 ;
 ; Enable the keyboard controller A20 gate
 ;
@@ -2381,13 +2377,31 @@ enable_a20:
                ; we don't want to *require* A20 masking (SYSLINUX should
                ; work fine without it, if the BIOS does.)
                push es
-               mov ax,0FFFFh           ; HMA
-               mov es,ax
-.a20_wait:     inc word [ss:A20test]
+               push cx
+               mov cx,0FFFFh           ; Times to try, also HMA
+               mov es,cx               ; HMA = segment 0FFFFh
+.a20_wait:     inc word [ss:A20Test]
                call try_wbinvd
-               mov ax,[es:A20test+10h]
-               cmp ax,[ss:A20test]
-               je .a20_wait
+               mov ax,[es:A20Test+10h]
+               cmp ax,[ss:A20Test]
+               jne .a20_done
+               loop .a20_wait
+;
+; Oh bugger.  A20 is not responding.  Try frobbing it again; eventually give up
+; and report failure to the user.
+;
+               pop cx
+               pop es
+
+               dec byte [ss:A20Tries]
+               jnz try_enable_a20
+
+               mov si, err_a20
+               jmp abort_load
+;
+; A20 unmasked, proceed...
+;
+.a20_done:     pop cx
                pop es
                ret
 
@@ -2399,9 +2413,9 @@ disable_a20:
 ;
 ; Disable the "fast A20 gate"
 ;
-               in al, 92h
+               in al, 092h
                and al,~02h
-               out 92h, al
+               out 092h, al
 ;
 ; Disable the keyboard controller A20 gate
 ;
@@ -2411,7 +2425,7 @@ disable_a20:
                call empty_8042
                mov al,0DDh             ; A20 off
                out 060h, al
-               jmp short kbc_delay
+               ; Fall through/tailcall to kbc_delay
 
 kbc_delay:     call empty_8042
                push cx
@@ -3383,6 +3397,7 @@ err_oldkernel   db 'Cannot load a ramdisk with an old kernel image.'
 err_notdos     db ': attempted DOS system call', 0Dh, 0Ah, 0
 err_comlarge   db 'COMBOOT image too large.', 0Dh, 0Ah, 0
 err_bootsec    db 'Invalid or corrupt boot sector image.', 0Dh, 0Ah, 0
+err_a20                db 0Dh, 0Ah, 'A20 gate not responding!', 0Dh, 0Ah, 0
 loading_msg     db 'Loading ', 0
 dotdot_msg      db '.'
 dot_msg         db '.', 0