PropBasic: Signed Two's complement?

skynugget

hi all, I'm trying to get the prop to talk to the roomba. i have it talking great, but i am having trouble reading the WORDS that are signed.

the roomba spits out two bytes per word highbyte first with a dec range of -32768 to 32768

i have come up with the following, but it only gives me the absolute value. does anyone have any ideas how i could make this work?

        RDBYTE rooBuffer(12), vDist
	vDist = vDist << 8 
	RDBYTE rooBuffer(13), temp
	vDist = vDist | temp

im using jons tx_dec routing to debug the output to the terminal. ive attached the rest of my code. thanks in advance!

Mike Green

You need to sign extend the 16 bit value. The easiest way to do this is to use 2 shifts like this:

vDist = vDist << 16 ' This positions the sign bit at bit 31
vDist = vDist ~> 16 ' This moves it back, but fills the upper word with the sign

Ariba

PropBasic does not know the ~> operator.
You can replace it with:

\  sar vDist,#16

but there is better alternative:

RDSBYTE rooBuffer(12), vDist
  vDist = vDist << 8
  RDBYTE rooBuffer(13), temp
  vDist = vDist | temp

This does the sign extend while reading the first byte

Andy

skynugget

thanks guys,

i tried both of those solutions before to no avail. either with the pasm instruction, or starting off with RDSBYTE results in absolute value.

kuroneko

skynugget wrote: »

i tried both of those solutions before to no avail. either with the pasm instruction, or starting off with RDSBYTE results in absolute value.

If I use RDSBYTE I get extra code added which deals with sign-extension. Do you mind attaching the generated SPIN code?

mov           __temp1,#12                  '	RDSBYTE rooBuffer(12), vDist
                  add           __temp1,#rooBuffer_ofs      
                  add           __temp1,par                 
                  rdbyte        vDist,__temp1               
                  shl           vDist,#24                   
                  sar           vDist,#24                   

                                                             '	vDist = vDist << 8 
                  shl           vDist,#8

skynugget

yep i get that code in my spin as well kuroneko. (attached). could it be something in the TX_DEC routine causing me trouble?

kuroneko

skynugget wrote: »

yep i get that code in my spin as well kuroneko. (attached). could it be something in the TX_DEC routine causing me trouble?

Possibly, what values do you get when you display both byte values separately (without sign extension)? Preferably for a negative word.

Bean

Try this code:

RDBYTE rooBuffer(12), vDist, temp
vDist = vDist << 8
vDist = vDist | temp
vDist = vDist << 16
\ SAR vDist,#16


If the bytes would be in the other order you would just use RDSWORD.

Bean

skynugget

hey all, taking a fresh look at it this morning i realized i was testing the variable wrong. The roomba only reports -dist if it does it under its own power, not by pushing it backwards as i was doing, doh! (im pretty sure ive had this revelation before, and forgotten)
it turns out the code works and im a bone head! thanks everyone!