[Flashforth-devel] Switching Endians -- swapbytes for FF18

[craig b. <dab...@ya...>]

I needed to swap bytes to send to another machine, so I implamented one of my favorite tricks from "back in the day" when I was learning MACRO-11 on the college  PDP-11/40...  Swapping register contents without auxiliary storage.  Mike's made it easy for us in the Pic18-s, probably the rest of the chips, too.

swapbytes ( xy -- yx ) \ reverses byte order on Top-Of-Stack

craig bair

---------------

\ Reverse ENDIAN-ness of 16-bit on Top Of Stack 
\ i.e. Swap Bytes 
\ ...minimalist approach, no temp storage loc 
\ 
\ needed constants (could be hardwired into code 
$ffe9 constant fsr0    \ ; 0xFE9 FSR0L 
\ $ffea constant fsr0h    \ ; 0xFEA FSR0H 
$ffef constant indf0    \ INDirect-by-FSR0 FileRegister 
1 constant f,     \ Destination File 
\ 
\ needed assembler constructor 
: as2 ( opcode "name" -- ) ( f a -- ) 
  co: 
  does> rot ic, or ic, ; 
\ 
\ needed assembler instructions 
$04 as3 decf,           ( f d a -- )   
$28 as3 incf,           ( f d a -- )   
$18 as3 xorwf,          ( f d a -- )   
$6e as2 movwf,          ( f a -- )     
\ 
\ actual word 
: swapbytes    ( xy -- yx ) \ swap Hi:Lo bytes on Top of Stack 
   [ indf0 w, a, movf,  ] 
   [ fsr0  f, a, decf,  ] 
   [ indf0 f, a, xorwf, ] 
   [ indf0 w, a, xorwf, ] 
   [ indf0 f, a, xorwf, ] 
   [ fsr0  f, a, incf,  ] 
   [ indf0 a, movwf,    ] 
; 


 
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