hello42.tbf1

( First step toward a tiny bootstrap Forth compiler written in itself.

Generates an ELF executable for x86 Linux that exits with return code
42, following Brian Raiter’s lead.

Compile-time primitives:
( — defines a comment that extends to the next right-paren
v — defines a dataspace label, like for a variable.
b — compiles a literal byte, numerically, into data space.
# — compiles a literal four-byte little-endian number into data space.
^ — the location where the program should start executing [everything else
    is just definitions]
space, newline — ignored

Run-time primitives:
W — given an address and size on the stack, writes the specified number of 
    bytes to stdout
Q — exits the program with a return code of 0
- — subtracts two numbers
@ — fetches a word from memory
! — stores to a word in memory
~ — bitwise-negates a word

Defined in this file, all necessarily run-time:
h — the ELF header
e — the location of the entry point word in the ELF header
o — the location of the word in the ELF program header that specifies the .ORG
S — the location of the word in the ELF program header that specifies filesize
$ — the end of the program
)

v h ( ELF header, Elf32_Ehdr )
( ELF info from http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html)
( but I'm using 4096 for my origin rather than 0x08048000)
    ( e_ident:) b 127 b 69 ( E) b 76 ( L) b 70 ( F) b 1 b 1 b 1
                b 0 b 0 b 0  b 0 b 0 b 0  b 0 b 0 b 0 ( 9 bytes of padding)
     ( e_type:) b 2 b 0   ( e_machine:) b 3 b 0
  ( e_version:) # 1         ( e_entry:) v e # 0
               ( The program header offset is 52, the same as the header size,
                  since the program header immediately follows this ELF header )
    ( e_phoff:) # 52        ( e_shoff:) # 0
    ( e_flags:) # 0        ( e_ehsize:) b 52 b 0 
( e_phentsize:) b 32 b 0    ( e_phnum:) b 1 b 0 
( e_shentsize:) b 40 b 0 ( e_shentnum:) b 0 b 0
 ( e_shstrndx:) b 0 b 0 

( program header, Elf32_Phdr; note that we are now 52 bytes from 'h' )
      ( p_type:) # 1       ( p_offset:) # 0 
     ( p_vaddr:) v o # 131072 ( p_paddr:) # 0 ( should be 0, not org as Brian has)
( Note that you can only make p_memsz as large as you want if p_flags has a 
  2 = PF_W in it.  Otherwise, even one extra byte results in a segfault. )
    ( p_filesz:) v S # 0    ( p_memsz:) # 655360 ( should be enough for anyone)
     ( p_flags:) # 7        ( p_align:) # 4096

( mov bl, 42) b 179 b 42
   ( inc eax) b 64
  ( int 0x80) b 205 b 128

v $

^ o @ 0 52 - - 0 32 - - e ! ( set entry point correctly wrt origin: o - -52 - -32 )
$  h - S ! ( subtract h from $ to get size of whole program )
h S @ W Q