XOR question

Don M · 2013-02-13 15:04

Using these data bytes as an example:

$01, $0A, $03, $00, $00

How do I code this to XOR these bytes for a checksum answer? I see the checksum on the logic trace is $08 and it figures with my calculator that way but I don't know how to put it into code.

Would it be as simple as chksum := ($01 ^ $0A ^ $03 ^ $00 ^ $00) ?

Don M · 2013-02-13 15:07

Actually that works. Is there a better or more accepted way?

Don M · 2013-02-13 15:22

How do I get this chksum calculation to work correctly?

PUB Demo  | d, i, s, chksum

  ser.Start(RX, TX, 0, 9_600)                        ' communicate with 4D display
  term.start(31, 30, %0000, 115_200)                    ' start terminal using PST
  outa[RST]~~
  dira[RST]~~                                           ' Set RST pin as output
   
  
  pause(500)
  term.str(@ver)                                        ' display version     


' Goto Form3

  repeat i from 0 to 4
    s := byte[@form3][i]
    ser.tx(s)
    chksum := (chksum ^ i)
  ser.tx(chksum)
  term.hex(chksum, 4)
  
DAT
form3   byte  $01, $0A, $03, $00, $00

It calculates wrong. It shows $04 instead of $08.

Don M · 2013-02-13 15:24

Sorry- i figured it out. My mistake.

PUB Demo  | d, i, s, chksum

  ser.Start(RX, TX, 0, 9_600)                        ' communicate with 4D display
  term.start(31, 30, %0000, 115_200)                    ' start terminal using PST
  outa[RST]~~
  dira[RST]~~                                           ' Set RST pin as output
   
  
  pause(500)
  term.str(@ver)                                        ' display version     


' Goto Form3

  repeat i from 0 to 4
    s := byte[@form3][i]
    ser.tx(s)
    chksum := (chksum ^ s)
  ser.tx(chksum)
  term.hex(chksum, 4)

Mike Green · 2013-02-13 15:27

You have to initialize chksum to zero. Global variables (VAR) are initialized to zero and the method return variable (RESULT) is initialized to zero, but the local variables (d, i, s, chksum) are not. They take on whatever was left over in the stack area from before the method (Demo) was called. Sometimes that's zero, but it's not guaranteed.

Don M · 2013-02-13 16:20

Ok. Thanks Mike.

Mark_T · 2013-02-13 17:35

While on the subject of XOR I can't resist mentioning the hoary old trick for swapping two variables without using temporary storage:

  a ^= b
  b ^= a
  a ^= b

(though this doesn't need XOR if your machine is 3-address or has a reverse-subtract instruction

  a -= b
  b += a
  a := b - a   ' reverse subtract

(Of course PASM is 2-address and I don't think has a reverse-subtract but I bet there's a way to do this
without XOR or an extra register - mini challenge)

Don M · 2013-02-14 06:28

This works too:

  chksum := 0 
  repeat i from 0 to 70
    s := byte[@text][i]
    ser.tx(s)
    chksum ^= s  
  ser.tx(chksum)

Ariba · 2013-02-14 08:30

And this should also work:

chksum := 0 
  repeat i from 0 to 70
    s := text[i]
    ser.tx(s)
    chksum ^= s  
  ser.tx(chksum)

Andy

XOR question

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