source: bootcd/isolinux/syslinux-6.03/core/adv.inc

;; -----------------------------------------------------------------------
;;
;;   Copyright 2007-2008 H. Peter Anvin - All Rights Reserved
;;
;;   This program is free software; you can redistribute it and/or modify
;;   it under the terms of the GNU General Public License as published by
;;   the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
;;   Boston MA 02110-1301, USA; either version 2 of the License, or
;;   (at your option) any later version; incorporated herein by reference.
;;
;; -----------------------------------------------------------------------

;;
;; adv.inc
;;
;; The auxillary data vector and its routines
;;
;; The auxillary data vector is a 512-byte aligned block that on the
;; disk-based derivatives can be part of the syslinux file itself.  It
;; exists in two copies; when written, both copies are written (with a
;; sync in between, if from the operating system.)  The first two
;; dwords are magic number and inverse checksum, then follows the data
;; area as a tagged array similar to BOOTP/DHCP, finally a tail
;; signature.
;;
;; Note that unlike BOOTP/DHCP, zero terminates the chain, and FF
;; has no special meaning.
;;

;;
;; List of ADV tags...
;;
ADV_BOOTONCE    equ 1

;;
;; Other ADV data...
;;
ADV_MAGIC1      equ 0x5a2d2fa5                  ; Head signature
ADV_MAGIC2      equ 0xa3041767                  ; Total checksum
ADV_MAGIC3      equ 0xdd28bf64                  ; Tail signature

ADV_LEN         equ 500                         ; Data bytes

adv_retries     equ 6                           ; Disk retries

                section .data
                global __syslinux_adv_ptr, __syslinux_adv_size
__syslinux_adv_ptr:
                dd adv0.data
__syslinux_adv_size:
                dd ADV_LEN

                section .adv
                ; Introduce the ADVs to valid but blank
adv0:
.head           resd 1
.csum           resd 1
.data           resb ADV_LEN
.tail           resd 1
.end            equ $
adv1:
.head           resd 1
.csum           resd 1
.data           resb ADV_LEN
.tail           resd 1
.end            equ $
                section .text16

                ;
                ; This is called after config file parsing, so we know
                ; the intended location of the ADV
                ;
                global adv_init
adv_init:
                cmp byte [ADVDrive],-1
                jne adv_read

%if IS_SYSLINUX || IS_EXTLINUX
                cmp word [ADVSectors],2         ; Not present?
                jb adv_verify

                mov eax,[Hidden]
                mov edx,[Hidden+4]
                add [ADVSec0],eax
                adc [ADVSec0+4],edx
                add [ADVSec1],eax
                adc [ADVSec1+4],edx
                mov al,[DriveNumber]
                mov [ADVDrive],al
                jmp adv_read
%endif

                ;
                ; Initialize the ADV data structure in memory
                ;
adv_verify:
                cmp byte [ADVDrive],-1          ; No ADV configured, still?
                je .reset                       ; Then unconditionally reset

                mov si,adv0
                call .check_adv
                jz .ok                          ; Primary ADV okay
                mov si,adv1
                call .check_adv
                jz .adv1ok

                ; Neither ADV is usable; initialize to blank
.reset:
                mov di,adv0
                mov eax,ADV_MAGIC1
                stosd
                mov eax,ADV_MAGIC2
                stosd
                xor eax,eax
                mov cx,ADV_LEN/4
                rep stosd
                mov eax,ADV_MAGIC3
                stosd

.ok:
                ret

                ; The primary ADV is bad, but the backup is OK
.adv1ok:
                mov di,adv0
                mov cx,512/4
                rep movsd
                ret


                ; SI points to the putative ADV; unchanged by routine
                ; ZF=1 on return if good
.check_adv:
                push si
                lodsd
                cmp eax,ADV_MAGIC1
                jne .done                       ; ZF=0, i.e. bad
                xor edx,edx
                mov cx,ADV_LEN/4+1              ; Remaining dwords
.csum:
                lodsd
                add edx,eax
                loop .csum
                cmp edx,ADV_MAGIC2
                jne .done
                lodsd
                cmp eax,ADV_MAGIC3
.done:
                pop si
                ret

;
; adv_get: find an ADV string if present
;
; Input:        DL      = ADV ID
; Output:       CX      = byte count (zero on not found)
;               SI      = pointer to data
;               DL      = unchanged
;
; Assumes CS == DS.
;

adv_get:
                push ax
                mov si,adv0.data
                xor ax,ax                       ; Keep AH=0 at all times
.loop:
                lodsb                           ; Read ID
                cmp al,dl
                je .found
                and al,al
                jz .end
                lodsb                           ; Read length
                add si,ax
                cmp si,adv0.tail
                jb .loop
                jmp .end

.found:
                lodsb
                mov cx,ax
                add ax,si                       ; Make sure it fits
                cmp ax,adv0.tail
                jbe .ok
.end:
                xor cx,cx
.ok:
                pop ax
                ret

;
; adv_set: insert a string into the ADV in memory
;
; Input:        DL      = ADV ID
;               FS:BX   = input buffer
;               CX      = byte count (max = 255!)
; Output:       CF=1 on error
;               CX      clobbered
;
; Assumes CS == DS == ES.
;
adv_set:
                push ax
                push si
                push di
                and ch,ch
                jnz .overflow

                push cx
                mov si,adv0.data
                xor ax,ax
.loop:
                lodsb
                cmp al,dl
                je .found
                and al,al
                jz .endz
                lodsb
                add si,ax
                cmp si,adv0.tail
                jb .loop
                jmp .end

.found:         ; Found, need to delete old copy
                lodsb
                lea di,[si-2]
                push di
                add si,ax
                mov cx,adv0.tail
                sub cx,si
                jb .nukeit
                rep movsb                       ; Remove the old one
                mov [di],ah                     ; Termination zero
                pop si
                jmp .loop
.nukeit:
                pop si
                jmp .end
.endz:
                dec si
.end:
                ; Now SI points to where we want to put our data
                pop cx
                mov di,si
                jcxz .empty
                add si,cx
                cmp si,adv0.tail-2
                jae .overflow                   ; CF=0

                mov si,bx
                mov al,dl
                stosb
                mov al,cl
                stosb
                fs rep movsb

.empty:
                mov cx,adv0.tail
                sub cx,di
                xor ax,ax
                rep stosb                       ; Zero-fill remainder

                clc
.done:
                pop di
                pop si
                pop ax
                ret
.overflow:
                stc
                jmp .done

;
; adv_cleanup:  checksum adv0 and copy to adv1
;               Assumes CS == DS == ES.
;
adv_cleanup:
                pushad
                mov si,adv0.data
                mov cx,ADV_LEN/4
                xor edx,edx
.loop:
                lodsd
                add edx,eax
                loop .loop
                mov eax,ADV_MAGIC2
                sub eax,edx
                lea di,[si+4]                   ; adv1
                mov si,adv0
                mov [si+4],eax                  ; Store checksum
                mov cx,(ADV_LEN+12)/4
                rep movsd
                popad
                ret

;
; adv_write:    write the ADV to disk.
;
;               Location is in memory variables.
;               Assumes CS == DS == ES.
;
;               Returns CF=1 if the ADV cannot be written.
;
                global adv_write
adv_write:
                push eax
                mov eax,[ADVSec0]
                or eax,[ADVSec0+4]
                je .bad
                mov eax,[ADVSec1]
                or eax,[ADVSec1+4]
                je .bad
                cmp byte [ADVDrive],-1
                je .bad

                call adv_cleanup
                mov ah,3                        ; Write
                call adv_read_write

                clc
                pop eax
                ret
.bad:                                           ; No location for ADV set
                stc
                pop eax
                ret

;
; adv_read:     read the ADV from disk
;
;               Location is in memory variables.
;               Assumes CS == DS == ES.
;
adv_read:
                push ax
                mov ah,2                        ; Read
                call adv_read_write
                call adv_verify
                pop ax
                ret

;
; adv_read_write: disk I/O for the ADV
;
;               On input, AH=2 for read, AH=3 for write.
;               Assumes CS == DS == ES.
;
adv_read_write:
                mov [ADVOp],ah
                pushad

                ; Check for EDD
                mov bx,55AAh
                mov ah,41h                      ; EDD existence query
                mov dl,[ADVDrive]
                int 13h
                mov si,.cbios
                jc .noedd
                cmp bx,0AA55h
                jne .noedd
                test cl,1
                jz .noedd
                mov si,.ebios
.noedd:

                mov eax,[ADVSec0]
                mov edx,[ADVSec0+4]
                mov bx,adv0
                call .doone

                mov eax,[ADVSec1]
                mov edx,[ADVSec1+4]
                mov bx,adv1
                call .doone

                popad
                ret

.doone:
                push si
                jmp si

.ebios:
                mov cx,adv_retries
.eb_retry:
                ; Form DAPA on stack
                push edx
                push eax
                push es
                push bx
                push word 1                     ; Sector count
                push word 16                    ; DAPA size
                mov si,sp
                pushad
                mov dl,[ADVDrive]
                mov ax,4000h
                or ah,[ADVOp]
                push ds
                push ss
                pop ds
                int 13h
                pop ds
                popad
                lea sp,[si+16]                  ; Remove DAPA
                jc .eb_error
                pop si
                ret
.eb_error:
                loop .eb_retry
                stc
                pop si
                ret

.cbios:
                push edx
                push eax
                push bp

                and edx,edx                     ; > 2 TiB not possible
                jnz .cb_overflow

                mov dl,[ADVDrive]
                and dl,dl
                ; Floppies: can't trust INT 13h 08h, we better know
                ; the geometry a priori, which means it better be our
                ; boot device...
                jns .noparm                     ; Floppy drive... urk

                mov ah,08h                      ; Get disk parameters
                int 13h
                jc .noparm
                and ah,ah
                jnz .noparm
                shr dx,8
                inc dx
                movzx edi,dx                    ; EDI = heads
                and cx,3fh
                movzx esi,cx                    ; ESI = sectors/track
                jmp .parmok

.noparm:
                ; No CHS info... this better be our boot drive, then
%if IS_SYSLINUX || IS_EXTLINUX
                cmp dl,[DriveNumber]
                jne .cb_overflow                ; Fatal error!
                movzx esi,word [bsSecPerTrack]
                movzx edi,word [bsHeads]
%else
                ; Not a disk-based derivative... there is no hope
                jmp .cb_overflow
%endif

.parmok:
                ;
                ; Dividing by sectors to get (track,sector): we may have
                ; up to 2^18 tracks, so we need to use 32-bit arithmetric.
                ;
                xor edx,edx
                div esi
                xor cx,cx
                xchg cx,dx              ; CX <- sector index (0-based)
                                        ; EDX <- 0
                ; eax = track #
                div edi                 ; Convert track to head/cyl

                ; Watch out for overflow, we might be writing!
                cmp eax,1023
                ja .cb_overflow

                ;
                ; Now we have AX = cyl, DX = head, CX = sector (0-based),
                ; BP = sectors to transfer, SI = bsSecPerTrack,
                ; ES:BX = data target
                ;

                shl ah,6                ; Because IBM was STOOPID
                                        ; and thought 8 bits were enough
                                        ; then thought 10 bits were enough...
                inc cx                  ; Sector numbers are 1-based, sigh
                or cl,ah
                mov ch,al
                mov dh,dl
                mov dl,[ADVDrive]
                mov al,01h              ; Transfer one sector
                mov ah,[ADVOp]          ; Operation

                mov bp,adv_retries
.cb_retry:
                pushad
                int 13h
                popad
                jc .cb_error

.cb_done:
                pop bp
                pop eax
                pop edx
                pop si
                ret

.cb_error:
                dec bp
                jnz .cb_retry
.cb_overflow:
                stc
                jmp .cb_done

                section .data16
                alignz 8
ADVSec0         dq 0                    ; Not specified
ADVSec1         dq 0                    ; Not specified
ADVDrive        db -1                   ; No ADV defined
ADVCHSInfo      db -1                   ; We have CHS info for this drive

                section .bss16
ADVOp           resb 1

                section .text16