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Brainfuck asm source for Linux

A tiny asm version of this simple language

(by brian raiter)

Brainfuck is a very simplistic programming language, which was invented by Urban Mueller solely for the purpose of being able to create a compiler that was less than 256 bytes in size, for the Amiga OS. (More information about Brainfuck can be found at http://www.muppetlabs.com/~breadbox/bf/.) I eventually decided to take up the challenge as well, and create a Brainfuck compiler for Linux using less than 256 bytes.
This article is online from 2518 days and has been seen 1973 times


;; bf.asm: Copyright (C) 1999-2001 by Brian Raiter, under the GNU
;; General Public License (version 2 or later). No warranty.
;;
;; To build:
;; nasm -f bin -o bf bf.asm && chmod +x bf
;; To use:
;; bf < foo.b > foo && chmod +x foo

BITS 32

;; This is the size of the data area supplied to compiled programs.

%define arraysize 30000

;; For the compiler, the text segment is also the data segment. The
;; memory image of the compiler is inside the code buffer, and is
;; modified in place to become the memory image of the compiled
;; program. The area of memory that is the data segment for compiled
;; programs is not used by the compiler. The text and data segments of
;; compiled programs are really only different areas in a single
;; segment, from the system's point of view. Both the compiler and
;; compiled programs load the entire file contents into a single
;; memory segment which is both writeable and executable.

%define TEXTORG 0x45E9B000
%define DATAOFFSET 0x2000
%define DATAORG (TEXTORG + DATAOFFSET)

;; Here begins the file image.

org TEXTORG

;; At the beginning of the text segment is the ELF header and the
;; program header table, the latter consisting of a single entry. The
;; two structures overlap for a space of eight bytes. Nearly all
;; unused fields in the structures are used to hold bits of code.

;; The beginning of the ELF header.

db 0x7F, "ELF" ; ehdr.e_ident

;; The top(s) of the main compiling loop. The loop jumps back to
;; different positions, depending on how many bytes to copy into the
;; code buffer. After doing that, esi is initialized to point to the
;; epilog code chunk, a copy of edi (the pointer to the end of the
;; code buffer) is saved in ebp, the high bytes of eax are reset to
;; zero (via the exchange with ebx), and then the next character of
;; input is retrieved.

emitputchar: add esi, byte (putchar - decchar) - 4
emitgetchar: lodsd
emit6bytes: movsd
emit2bytes: movsb
emit1byte: movsb
compile: lea esi, [byte ecx + epilog - filesize]
xchg eax, ebx
cmp eax, 0x00030002 ; ehdr.e_type (0x0002)
; ehdr.e_machine (0x0003)
mov ebp, edi ; ehdr.e_version
jmp short getchar

;; The entry point for the compiler (and compiled programs), and the
;; location of the program header table.

dd _start ; ehdr.e_entry
dd proghdr - $$ ; ehdr.e_phoff

;; The last routine of the compiler, called when there is no more
;; input. The epilog code chunk is copied into the code buffer. The
;; text origin is popped off the stack into ecx, and subtracted from
;; edi to determine the size of the compiled program. This value is
;; stored in the program header table, and then is moved into edx.
;; The program then jumps to the putchar routine, which sends the
;; compiled program to stdout before falling through to the epilog
;; routine and exiting.

eof: movsd ; ehdr.e_shoff
xchg eax, ecx
pop ecx
sub edi, ecx ; ehdr.e_flags
xchg eax, edi
stosd
xchg eax, edx
jmp short putchar ; ehdr.e_ehsize

;; 0x20 == the size of one program header table entry.

dw 0x20 ; ehdr.e_phentsize

;; The beginning of the program header table. 1 == PT_LOAD, indicating
;; that the segment is to be loaded into memory.

proghdr: dd 1 ; ehdr.e_phnum & phdr.p_type
; ehdr.e_shentsize
dd 0 ; ehdr.e_shnum & phdr.p_offset
; ehdr.e_shstrndx

;; (Note that the next four bytes, in addition to containing the first
;; two instructions of the bracket routine, also comprise the memory
;; address of the text origin.)

db 0 ; phdr.p_vaddr

;; The bracket routine emits code for the "[" instruction. This
;; instruction translates to a simple "jmp near", but the target of
;; the jump will not be known until the matching "]" is seen. The
;; routine thus outputs a random target, and pushes the location of
;; the target in the code buffer onto the stack.

bracket: mov al, 0xE9
inc ebp
push ebp ; phdr.p_paddr
stosd
jmp short emit1byte

;; This is where the size of the executable file is stored in the
;; program header table. The compiler updates this value just before
;; it outputs the compiled program. This is the only field in the two
;; headers that differs between the compiler and its compiled
;; programs. (While the compiler is reading input, the first byte of
;; this field is also used as an input buffer.)

filesize: dd compilersize ; phdr.p_filesz

;; The size of the program in memory. This entry creates an area of
;; bytes, arraysize in size, all initialized to zero, starting at
;; DATAORG.

dd DATAOFFSET + arraysize ; phdr.p_memsz

;; The code chunk for the "." instruction. eax is set to 4 to invoke
;; the write system call. ebx, the file handle to write to, is set to
;; 1 for stdout. ecx points to the buffer containing the bytes to
;; output, and edx equals the number of bytes to output. (Note that
;; the first byte of the first instruction, which is also the least
;; significant byte of the p_flags field, encodes to 0xB3. Having the
;; 2-bit set marks the memory containing the compiler, and its
;; compiled programs, as writeable.)

putchar: mov bl, 1 ; phdr.p_flags
mov al, 4
int 0x80 ; phdr.p_align

;; The epilog code chunk. After restoring the initialized registers,
;; eax and ebx are both zero. eax is incremented to 1, so as to invoke
;; the exit system call. ebx specifies the process's return value.

epilog: popa
inc eax
int 0x80

;; The code chunks for the ">", "<", "+", and "-" instructions.

incptr: inc ecx
decptr: dec ecx
incchar: inc byte [ecx]
decchar: dec byte [ecx]

;; The main loop of the compiler continues here, by obtaining the next
;; character of input. This is also the code chunk for the ","
;; instruction. eax is set to 3 to invoke the read system call. ebx,
;; the file handle to read from, is set to 0 for stdin. ecx points to
;; a buffer to receive the bytes that are read, and edx equals the
;; number of bytes to read.

getchar: mov al, 3
xor ebx, ebx
int 0x80

;; If eax is zero or negative, then there is no more input, and the
;; compiler proceeds to the eof routine.

or eax, eax
jle eof

;; Otherwise, esi is advanced four bytes (from the epilog code chunk
;; to the incptr code chunk), and the character read from the input is
;; stored in al, with the high bytes of eax reset to zero.

lodsd
mov eax, [ecx]

;; The compiler compares the input character with ">" and "<". esi is
;; advanced to the next code chunk with each failed test.

cmp al, '>'
jz emit1byte
inc esi
cmp al, '<'
jz emit1byte
inc esi

;; The next four tests check for the characters "+", ",", "-", and
;; ".", respectively. These four characters are contiguous in ASCII,
;; and so are tested for by doing successive decrements of eax.

sub al, '+'
jz emit2bytes
dec eax
jz emitgetchar
inc esi
inc esi
dec eax
jz emit2bytes
dec eax
jz emitputchar

;; The remaining instructions, "[" and "]", have special routines for
;; emitting the proper code. (Note that the jump back to the main loop
;; is at the edge of the short-jump range. Routines below here
;; therefore use this jump as a relay to return to the main loop;
;; however, in order to use it correctly, the routines must be sure
;; that the zero flag is cleared at the time.)

cmp al, '[' - '.'
jz bracket
cmp al, ']' - '.'
relay: jnz compile

;; The endbracket routine emits code for the "]" instruction, as well
;; as completing the code for the matching "[". The compiler first
;; emits "cmp dh, [ecx]" and the first two bytes of a "jnz near". The
;; location of the missing target in the code for the "[" instruction
;; is then retrieved from the stack, the correct target value is
;; computed and stored, and then the current instruction's jmp target
;; is computed and emitted.

endbracket: mov eax, 0x850F313A
stosd
lea esi, [byte edi - 8]
pop eax
sub esi, eax
mov [eax], esi
sub eax, edi
stosd
jmp short relay

;; This is the entry point, for both the compiler and its compiled
;; programs. The shared initialization code sets eax and ebx to zero,
;; ecx to the beginning of the array that is the compiled programs's
;; data area, and edx to one. (This also clears the zero flag for the
;; relay jump below.) The registers are then saved on the stack, to be
;; restored at the very end.

_start:
xor eax, eax
xor ebx, ebx
mov ecx, DATAORG
cdq
inc edx
pusha

;; At this point, the compiler and its compiled programs diverge.
;; Although every compiled program includes all the code in this file
;; above this point, only the eleven bytes directly above are actually
;; used by both. This point is where the compiler begins storing the
;; generated code, so only the compiler sees the instructions below.
;; This routine first modifies ecx to contain TEXTORG, which is stored
;; on the stack, and then offsets it to point to filesize. edi is set
;; equal to codebuf, and then the compiler enters the main loop.

codebuf:
mov ch, (TEXTORG >> 8) & 0xFF
push ecx
mov cl, filesize - $$
lea edi, [byte ecx + codebuf - filesize]
jmp short relay

;; Here ends the file image.

compilersize equ $ - $$


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