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boot12.asm
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boot12.asm
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; ;;
;; "BootProg" Loader v 2.0 by Alexey Frunze (c) 2000-2023 ;;
;; 2-clause BSD license. ;;
;; ;;
;; ;;
;; How to Compile: ;;
;; ~~~~~~~~~~~~~~~ ;;
;; nasm boot12.asm -f bin -o boot12.bin ;;
;; ;;
;; ;;
;; Features: ;;
;; ~~~~~~~~~ ;;
;; - Loads a 16-bit executable file in the MS-DOS .COM or .EXE format ;;
;; from the root directory of a disk and transfers control to it ;;
;; (the "ProgramName" variable holds the name of the file to be loaded) ;;
;; ;;
;; - Prints an error if the file isn't found or couldn't be read ;;
;; (the "RE" and "ReadErr" messages stand for "Read Error", ;;
;; the "NF" and "NoFile" messages stand for "No File"/"file Not Found") ;;
;; ;;
;; - FAT12 supported, best suited for floppies ;;
;; ;;
;; - Supports i8086/i8088 CPUs. ;;
;; ;;
;; ;;
;; Known Limitations: ;;
;; ~~~~~~~~~~~~~~~~~~ ;;
;; - Works only on the 1st MBR partition which must be a PRImary DOS ;;
;; partition with FAT12 (File System ID: 1) ;;
;; ;;
;; ;;
;; Known Bugs: ;;
;; ~~~~~~~~~~~ ;;
;; - All bugs are fixed as far as I know. The boot sector has been tested ;;
;; on the following types of diskettes: ;;
;; - 360KB 5"25 ;;
;; - 1.2MB 5"25 ;;
;; - 1.44MB 3"5 ;;
;; ;;
;; ;;
;; Memory Layout: ;;
;; ~~~~~~~~~~~~~~ ;;
;; The diagram below shows the typical memory layout. The actual location ;;
;; of the boot sector and its stack may be lower than A0000H if the BIOS ;;
;; reserves memory for its Extended BIOS Data Area just below A0000H and ;;
;; reports less than 640 KB of RAM via its Int 12H function. ;;
;; ;;
;; physical address ;;
;; +------------------------+ 00000H ;;
;; | Interrupt Vector Table | ;;
;; +------------------------+ 00400H ;;
;; | BIOS Data Area | ;;
;; +------------------------+ 00500H ;;
;; | PrtScr Status / Unused | ;;
;; +------------------------+ 00600H <- ImageLoadSeg * 16 ;;
;; | Loaded Image | ;;
;; +------------------------+ nnnnnH ;;
;; | Available Memory | ;;
;; +------------------------+ A0000H - 8KB - 512 ;;
;; | 6KB FAT12 | ;;
;; +------------------------+ A0000H - 2KB - 512 ;;
;; | Boot Sector | ;;
;; +------------------------+ A0000H - 2KB ;;
;; | 2KB Boot Stack | ;;
;; +------------------------+ A0000H ;;
;; | Video RAM | ;;
;; ;;
;; ;;
;; Boot Image Startup (register values): ;;
;; ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ;;
;; dl = BIOS boot drive number (e.g. 0, 80H) ;;
;; cs:ip = program entry point ;;
;; ss:sp = program stack (don't confuse with boot sector's stack) ;;
;; COM program defaults: cs = ds = es = ss = 50h, sp = 0, ip = 100h ;;
;; EXE program defaults: ds = 60h, other stuff depends on EXE header ;;
;; Magic numbers: ;;
;; si = 16381 (prime number 2**14-3) ;;
;; di = 32749 (prime number 2**15-19) ;;
;; bp = 65521 (prime number 2**16-15) ;;
;; The magic numbers let the program know whether it has been loaded by ;;
;; this boot sector or by MS-DOS, which may be handy for universal, bare- ;;
;; metal and MS-DOS programs. ;;
;; ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
BITS 16
CPU 8086
%ifndef LOADSEG
%define LOADSEG 60h
%endif
%ifnum LOADSEG
%else
%error "LOADSEG isn't a number"
%endif
ImageLoadSeg equ LOADSEG
%ifndef STACKSIZE
%define STACKSIZE 2048
%endif
%ifnum STACKSIZE
%else
%error "STACKSIZE isn't a number"
%endif
StackSize equ STACKSIZE
%ifndef PROGNAME
%define PROGNAME "STARTUP BIN"
%endif
%ifstr PROGNAME
%strlen PROGNAMELEN PROGNAME
%if PROGNAMELEN != 11
%error "PROGNAME isn't an 11-char string"
%endif
%else
%error "PROGNAME isn't a string"
%endif
ORG 0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Boot sector starts here ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Base:
ParasPerSector: ; 2 bytes overwritten with data here
jmp short start ; MS-DOS/Windows checks for this jump
nop
bsOemName DB "BootProg" ; 0x03
;;;;;;;;;;;;;;;;;;;;;
;; BPB starts here ;;
;;;;;;;;;;;;;;;;;;;;;
bpbBytesPerSector DW 0 ; 0x0B
bpbSectorsPerCluster DB 0 ; 0x0D
bpbReservedSectors DW 0 ; 0x0E
bpbNumberOfFATs DB 0 ; 0x10
bpbRootEntries DW 0 ; 0x11
bpbTotalSectors DW 0 ; 0x13
bpbMedia DB 0 ; 0x15
bpbSectorsPerFAT DW 0 ; 0x16
bpbSectorsPerTrack DW 0 ; 0x18
bpbHeadsPerCylinder DW 0 ; 0x1A
bpbHiddenSectors DD 0 ; 0x1C
bpbTotalSectorsBig DD 0 ; 0x20
;;;;;;;;;;;;;;;;;;;
;; BPB ends here ;;
;;;;;;;;;;;;;;;;;;;
bsDriveNumber DB 0 ; 0x24
bsUnused DB 0 ; 0x25
bsExtBootSignature DB 0 ; 0x26
bsSerialNumber DD 0 ; 0x27
bsVolumeLabel DB "NO NAME " ; 0x2B
bsFileSystem DB "FAT12 " ; 0x36
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Boot sector code starts here ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
start:
cld
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; How much RAM is there? ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
int 12h ; get conventional memory size (in KBs)
mov cl, 6
shl ax, cl ; and convert it to 16-byte paragraphs
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reserve memory for the boot sector and its stack ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
sub ax, (512 + StackSize) / 16 ; reserve space for code and stack
mov es, ax ; intending (cs=ds=ss):0 -> top - 512 - StackSize
cli ; ss:sp change protection for 8088
mov ss, ax
mov sp, 512 + StackSize
sti
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Copy ourselves to top of memory ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov cx, 256
mov si, 7C00h
xor di, di
mov ds, di
rep movsw
;;;;;;;;;;;;;;;;;;;;;;
;; Jump to the copy ;;
;;;;;;;;;;;;;;;;;;;;;;
push es
mov cl, main
push cx
retf
main:
push cs
pop ds
mov [bsDriveNumber], dl ; store BIOS boot drive number
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Get drive parameters (HDD only). ;;
;; If a drive is moved between different systems, ;;
;; its geometry may differ in the eyes of the ;;
;; different BIOSes, more so when it's a disk image ;;
;; for a VM. ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
or dl, dl
jns NoNewGeometry ; skip if it's a floppy (dl < 80h)
mov ah, 8 ; clobbers AX,BL,CX,DX,ES:DI
int 13h
jc NoNewGeometry ; use the above BPB if no BIOS info
and cx, 63
mov [bpbSectorsPerTrack], cx
mov cl, dh
inc cx
mov [bpbHeadsPerCylinder], cx
NoNewGeometry:
xor bx, bx ; buffer offset; also for shorter encoding of memory operands
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reserve memory for the FAT12 image (6KB max) ;;
;; and load it in its entirety ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov ax, [bx+(bpbBytesPerSector-Base)]
mov cl, 4
shr ax, cl ; ax = sector size in paragraphs
mov [bx+(ParasPerSector-Base)], ax
mov cx, [bx+(bpbSectorsPerFAT-Base)] ; cx = FAT size in sectors
mul cx ; ax = FAT size in paragraphs
mov di, ss
sub di, ax
mov es, di ; es:bx -> buffer for the FAT
push es ; push FAT segment (2nd stack parameter)
xor ax, ax
cwd
; dx:ax = LBA of 1st FAT (relative to 1st FAT start)
call ReadSector ; cx = 0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Load the root directory in its entirety ;;
;; (16KB max) ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov ax, 32
mul word [bx+(bpbRootEntries-Base)]
div word [bx+(bpbBytesPerSector-Base)]
xchg cx, ax ; cx = root directory size in sectors
; ax = 0
mov al, [bx+(bpbNumberOfFATs-Base)]
mul word [bx+(bpbSectorsPerFAT-Base)]
; dx:ax = FATs size in sectors =
; LBA of root dir (relative to 1st FAT start)
mov di, ImageLoadSeg
mov es, di ; es:bx -> buffer for root dir & file
push es
call ReadSector ; cx = 0
pop es
push dx
push ax ; FATs size + root dir size in sectors (1st stack parameter)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Look for the COM/EXE file to load and run ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov di, bx ; es:di -> root entries array
mov dx, [bx+(bpbRootEntries-Base)]
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Looks for a file/dir by its name ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Input: ES:DI -> root directory array ;;
;; DX = number of root entries ;;
;; CH = 0 ;;
;; Output: DI = cluster number ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
FindName:
mov si, ProgramName ; ds:si -> program name
mov cl, 11 ; length of ProgramName
cmp byte [es:di], ch
je FindNameFailed ; end of root directory
push di
repe cmpsb
pop di
je FindNameFound
add di, 32
dec dx
jnz FindName ; next root entry
FindNameFailed:
call Error
%ifndef SHORTERR
db "NoFile", 0
%else
db "NF"
%endif
FindNameFound:
mov di, [es:di+1Ah] ; di = cluster no.
push es ; ImageLoadSeg
;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Load the entire file ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reads a FAT12 cluster ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Input: ES:BX -> buffer address ;;
;; BX = 0 ;;
;; DI = cluster no ;;
;; dword [SP+2] = FATs size + root dir size (in sectors) ;;
;; word [SP+6] = FAT segment ;;
;; Output: DI = next cluster ;;
;; ES:BX -> next addr (new ES:old BX) ;;
;; BX = 0 ;;
;; Clobbers: AX, CX, DX, BP ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ReadCluster:
mov bp, sp
lea ax, [di-2]
xor ch, ch
mov cl, [bx+(bpbSectorsPerCluster-Base)]
; cx = sector count
mul cx
add ax, [bp+2]
adc dx, [bp+4]
; dx:ax = cluster LBA
call ReadSector ; read 1 cluster
mov ax, 3
mul di
shr ax, 1
xchg ax, di ; di = cluster * 3 / 2
push ds
mov ds, [bp+6] ; ds = FAT segment
mov di, [di] ; di = next cluster
pop ds
mov cl, 4
jc ReadClusterOdd
shl di, cl ; will mask cluster value
ReadClusterOdd:
shr di, cl
cmp di, 0FF8h
jc ReadCluster ; if not End Of File
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Type detection, .COM or .EXE? ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
cli ; ss:sp change protection for 8088
mov dl, [bx+(bsDriveNumber-Base)] ; pass the BIOS boot drive
pop ax ; ImageLoadSeg
mov ds, ax ; ax=ds=seg the file is loaded to
cmp word [bx], 5A4Dh ; "MZ" signature?
je RelocateEXE ; yes, it's an EXE program
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Setup and run a .COM program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
sub ax, 10h ; "org 100h" stuff :)
mov es, ax
mov ds, ax
mov ss, ax
xor sp, sp
mov bh, 1 ; ax:bx = cs:ip of entry point
jmp short Run
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Relocate, setup and run a .EXE program ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
RelocateEXE:
add ax, [bx+08h] ; ax = image base
mov cx, [bx+06h] ; cx = reloc items
mov si, [bx+18h] ; si = reloc table pointer
jcxz RelocationDone
ReloLoop:
add [si+2], ax ; item seg (rel):ofs -> item seg (abs):ofs
les di, [si] ; es:di = item seg (abs):ofs
add [es:di], ax ; fixup item
add si, 4 ; point to next entry
loop ReloLoop
RelocationDone:
mov cx, ax
add cx, [bx+0Eh]
mov ss, cx ; ss for EXE
mov sp, [bx+10h] ; sp for EXE
add ax, [bx+16h]
mov bx, [bx+14h] ; ax:bx = cs:ip of entry point
Run:
push ax
push bx
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Set the magic numbers so the program knows that it ;;
;; has been loaded by this bootsector and not by MS-DOS ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
mov si, 16381 ; prime number 2**14-3
mov di, 32749 ; prime number 2**15-19
mov bp, 65521 ; prime number 2**16-15
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; All done, transfer control to the program now ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
sti
retf
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Reads a sector using BIOS Int 13h fn 2 ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Input: DX:AX = LBA (relative to 1st FAT start) ;;
;; CX = sector count ;;
;; ES:BX -> buffer address ;;
;; BX = 0 ;;
;; Output: DX:AX = next LBA ;;
;; CX = 0 ;;
;; ES:BX -> next addr (new ES:old BX) ;;
;; BX = 0 ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ReadSector:
push di
ReadSectorNext:
mov di, 5 ; attempts to read
ReadSectorRetry:
push ax
push dx
push cx
add ax, [bx+(bpbReservedSectors-Base)]
adc dx, bx ; Can't overflow (bpbTotalSectors(Big) includes bpbReservedSectors).
add ax, [bx+(bpbHiddenSectors-Base)]
adc dx, [bx+(bpbHiddenSectors-Base)+2]
; Limit the LBA to 24 bits, which should avoid most #DEs
; (but probably not all) in the following divisions in case
; this FAT12 volume isn't fully CHS-addressable.
jc ReadError
test dh, dh
jnz ReadError
; Divide 32-bit LBA in dx:ax by 16-bit bpbSectorsPerTrack
; with 32-bit quotient and 16-bit remainder.
; This avoids division overflows with large LBAs and
; supports disks up to 8 GB in size (the BIOS int 13h read
; function (ah = 2) takes cylinder:head:sector (AKA CHS)
; that's at most 24-bit, IOW, the function takes LBAs up to
; ~16 million, which with 512-byte sectors gives 8GB).
xchg ax, cx ; save LBA's low word in cx
xchg ax, dx ; will first divide LBA's hi word
cwd
div word [bx+(bpbSectorsPerTrack-Base)]
; ax = (LBA / 65536) / SPT = (LBA / SPT) / 65536
; dx = (LBA / 65536) % SPT
xchg ax, cx ; will next divide LBA's low word
div word [bx+(bpbSectorsPerTrack-Base)]
; cx:ax = LBA / SPT
; dx = LBA % SPT = sector - 1
xchg cx, dx
inc cx
; cx = sector no.
div word [bx+(bpbHeadsPerCylinder-Base)]
; ax = (LBA / SPT) / HPC = cylinder
; dx = (LBA / SPT) % HPC = head
mov ch, al
; ch = LSB 0...7 of cylinder no.
cmp ah, 4
jae ReadError ; limit cylinder no. to less than 1024
ror ah, 1
ror ah, 1 ; excess bits of cylinder no. must be 0 anyway
or cl, ah
; cl = MSB 8...9 of cylinder no. + sector no.
mov dh, dl
; dh = head no.
mov ax, 201h ; clobbers AX
; al = sector count = 1
; ah = 2 = read function no.
mov dl, [bx+(bsDriveNumber-Base)]
; dl = drive no.
int 13h ; read sectors
%ifdef INJ_ERROR
stc
%endif
%ifdef INJ_RETRY
call InjectReadRetry
%endif
jnc ReadSectorDone ; CF = 0 if no error
mov ah, 0 ; clobbers AX
; ah = 0 = reset function
int 13h ; reset drive
pop cx
pop dx
pop ax
dec di
jnz short ReadSectorRetry ; extra attempt
ReadError:
call Error
%ifndef SHORTERR
db "ReadErr", 0
%else
db "RE"
%endif
ReadSectorDone:
mov ax, es
add ax, [bx+(ParasPerSector-Base)]
mov es, ax ; es updated
pop cx
pop dx
pop ax
inc ax
jnz ReadSectorIncd
inc dx
ReadSectorIncd: ; adjust LBA for next sector
loop ReadSectorNext ; if not last sector
pop di
ret
%ifdef INJ_RETRY
;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Read retry injection ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;
InjectReadRetry:
jc InjectReadRetryDone
cmp di, 2 ; carry=1 IFF di<2
cmc ; carry=1 IFF di>=2
InjectReadRetryDone:
ret
%endif
;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Error Messaging Code ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;
Error:
pop si
mov ah, 0Eh
mov bx, 7
%ifndef SHORTERR
ErrorNext:
lodsb
Halt:
hlt
test al, al
jz Halt
int 10h
jmp short ErrorNext
%else
lodsb
int 10h ; 1st char
lodsb
int 10h ; 2nd/last char
Halt:
hlt
jmp short Halt
%endif
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Fill free space with zeroes ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
times (512-13-($-$$)) db 0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Name of the file to load and run ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
ProgramName db PROGNAME ; name and extension each must be
; padded with spaces (11 bytes total)
;;;;;;;;;;;;;;;;;;;;;;;;;;
;; End of the sector ID ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;
dw 0AA55h ; BIOS checks for this ID