Learning Forth: Better way to increment variables

[JonnyBritish]

In my code I am tracking a sprites position as a variable called AY. I wanted to add 1 to the variable so used this TurboForth code which is perfectly valid but is 5 Forth words long

 

AY @ 1+ AY !

 

Here is how it works

 

AY @ - retrieve valiable AY and store on the stack

1+ - retrieve top most cell from stack and add 1 to the value and place back on the stack

AY ! - retrieve the top most value on the stack and place back in variable AY

 

However there is a better way which is only 3 Forth words long and that is

 

1 AY +!

 

From the docs at http://www.turboforth.net/language_reference.html

and specifically

 

+! value address -- S Read the cell at address, adds value to it and stores the result at address.

 

You can see it combines all the operations into a 3 word line.

 

same for subtraction

 

-! value address -- S Reads the cell at address. Subtracts value from the cell value, and stores the result at address.

[moulinaie]

 

However there is a better way which is only 3 Forth words long and that is

 

1 AY +!

 

I worked on a FORTH version on the Atari ST with a word: 1+!

 

so it would be AY 1+!, only two words

 

Guillaume.

[Willsy]

Hmmm.... Good idea...

 

You mean like this:

 

ASM: 1+! ( address -- )
*SP R0 MOV, \ get address in r0
R0 ** INC,  \ increment the value at that address
SP DECT,	\ pop address
;ASM

ASM: 1-! ( address -- )
*SP R0 MOV, \ get address in r0
R0 ** DEC,  \ decrement the value at that address
SP DECT,	\ pop address
;ASM

 

 

You can paste that right in to classic99 - just load block 9 first (9 LOAD) to load the assembler.

 

Let's convert those into CODE words (so that we don't need to load the assembler every time to use them):

 

29 LOAD
ASM>CODE 1+! CLIP
ASM>CODE 1-! CLIP

 

Here I have loaded ASM>CODE from block 29 which converts assembly words that are already in the dictionary to CODE words, and I used the device CLIP for an output device which is a classic99 only device, so ASM>CLIP will send it's output to the windows clipboard.

 

And hey-presto, the following is in the windows clipboard:

 

CODE: 1+!
C014 0590 0644 ;CODE

CODE: 1-!
C014 0610 0644 ;CODE

 

And there you go. You paste/type those into a block, and you have them forever more. Just load them when you need them! (Don't forget to add the word HEX before the CODE: word and DECIMAL at the end - I'll probably make CODE: set HEX automatically in a future version, and restore the base to the previous base at the end). So, you end up with:

 

HEX
CODE: 1+!
C014 0590 0644 ;CODE

CODE: 1-!
C014 0610 0644 ;CODE
DECIMAL

 

Let's test them:

 

$100 $B000 ! ok:0

$B000 1+! ok:0

$B000 @ $. 101 ok:0

$B000 1-! ok:0

$B000 @ $. 100 ok:0

 

 

Or, with a variable:

 

VARIABLE TEST ok:0

100 TEST ! ok:0

TEST 1+! TEST @ . 101 ok:0

TEST 1-! TEST @ . 100 ok:0

 

Et Voilà