:ASM overwrite
  MOV AH 0x3c #
  XOR CX CX ( non-system, non-hidden )
  POP DX ( filename ptr )
  INT 0x21 #
  PUSH AX
  NEXT

:ASM open
  MOV AH 0x3d #
  MOV AL 2 # ( read/write access, allow child inheritance )
  POP DX ( filename ptr )
  INT 0x21 #
  PUSH AX
  NEXT

:ASM openself ( -- fp )
  MOV AX 0x2c @+ ( fetch environment segment )
  OR AX AX
  JZ 1 @> ( we're in a target .com and there is no "self" - abort with 0 )
  MOV ES AX
  XOR DI DI
  XOR AX AX
0 :> ( search for two zeroes in a row )
  SCASB
  JNZ 0 <@
  SCASB
  JNZ 0 <@
  SCASW ( consume the 0x0001. DI is now a pointer to the filename )
  MOV DX DI
  PUSH DS
  MOV AX ES
  MOV DS AX
  MOV AH 0x3d #
  MOV AL 2 # ( read/write access, allow child inheritance )
  INT 0x21 #
  POP DS
1 <:
  PUSH AX
  NEXT

:ASM close
  MOV AH 0x3e #
  POP BX
  INT 0x21 #
  NEXT

:ASM seekto ( loc fp -- )
  XOR AX AX
  XOR CX CX
  MOV AH 0x42 #
  POP BX
  POP DX
  INT 0x21 #
  NEXT

0 VAR, fcount
:ASM fread
  MOV AH 0x3f #
  POP BX ( fp )
  POP DX ( buffer )
  POP CX ( length )
  INT 0x21 #
  MOV t& fcount @+ AX ( save number of bytes read )
  NEXT

:ASM fwrite
  MOV AH 0x40 #
  POP BX ( fp )
  POP DX ( buffer )
  POP CX ( length )
  INT 0x21 #
  MOV t& fcount @+ AX ( save number of bytes written )
  NEXT

-1 CONST EOF
0 VAR, fbuffer
:t fgetc ( fp -- c )
  1 fbuffer <rot fread fbuffer ub@
  fcount @ not BZ_ [ patchpt ] drop EOF [ patch!t ] ;
:t fputc ( c fp -- ) swap fbuffer b! 1 fbuffer <rot fwrite ;