Two's compliment in PBasic?

lindseydallas

Hello,

Is there a command in PBasic to do a two's compliment on a binary number? Has anyone else had to do this?

Two's compliment is when you take a binary number and switch the 0s to 1s and the 1s to 0s and then add one.
For example:
Take
10101010 then do the switch
01010101 then add one
01010110 gives the two's compliment

Any ideas?
Thanks!

Mike Green

A minus sign will do it. See page 105-106 of the reference manual.

Like this: B = -A

There's also an operator to do one's complement where you change 0's to 1's and vice versa. It's ~ (tilde), so you can do the two separate steps also.

Like this: B = ~A +1

The results are the same.

Steph Lindsay

I don't think there's a single command for that, but would the REV operator get you halfway there?

Steph Lindsay

Thanks Mike!

Tracy Allen

There are quite a few twos compliment helpers in PBASIC.

For example, the following snippet takes a number, then applies the ones compliment operator ~ (tilda) and then adds one to get the twos complement. That is just the negative of the number and could be written -N with the plain ordinary old minus sign. On output, the DEC modifier shows numbers as positive from 0 to 65535, whereas the SDEC modifier treats the number as twos complement from -32768 to +32767.

[SIZE=1][FONT=courier new]' {$STAMP BS2pe}
' {$PBASIC 2.5}

N VAR WORD
M VAR WORD

Main:
  N = 12345
  M = ~N + 1   ' ones complement and add 1, same as negative N
  DEBUG CR, DEC M, CR, SDEC M, CR, SDEC M+N, CR[/FONT][/SIZE]

What more do you want to accomplish with it? PBASIC treats numbers as twos complement for operations of addition, subtraction and multiplication. Division and modulus are a separate issue.

Mike Green

REV reverses the order of the bits in the number, but doesn't otherwise change the bits. The reference manual has all the details. Make sure you have a copy on hand. You can download it here if you don't have a paper copy.

lindseydallas

LOVE simple solutions! The negative sign worked perfectly.
I am using it to find the LRC for a Modbus ASCII message so I don't have to do anything fancy with it; just send it as a hex through the Serout command.

Sorry I didn't think to look in the manual.

Thanks!