科普向——使用BrainFuck语言编写简单网页

;; bf.asm: Copyright (C) 1999 Brian Raiter <breadbox@muppetlabs.com>
;; Licensed under the terms of the GNU General Public License, either
;; version 2 or (at your option) any later version.
;;
;; 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 ecx to the beginning
;; of the array that is the compiled program'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 end.

_start:
        mov ecx, DATAORG
        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 three instructions 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 $ - $$