;; 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	$ - $$