Comment by dotancohen

The FORTH-83 standard changed FIG-FORTH's official value of TRUE from 1 to -1 so all its bits were set. That was a rough transition like Python 2=>3, but worth it. It also defined /MOD integer division to be floored (rounded towards -infinity instead of zero like FIG-FORTH), which was also a tough change but the right one, especially for graphics.

>4. For various reasons the definition of all divide functions general effect is that quotients are floored instead of rounded toward zero. This should cause no problems for most pre-existing application software. The new divide functions are marginally slower than the old (a few machine cycles under most circumstances). The side-effects of the redefinition for floored divide can be counter-intuitive under some circumstances. For example, in FIG-Forth the operation

      -40 360 MOD

>would return the obvious answer (-40) on the stack, while 83- Standard Forth will return the answer 320!

>5. The true flag returned by all logical operations has been changed from the value 1 (in FIG-Forth) to the value -1 (in Forth-83, all bits set). If your code used the 0 or 1 returned by a comparison in an arithmetic operation, you will need to interpolate the operator ABS after the logical operator. This is a particularly difficult problem to look for in your source code. However, we feel that this mutation in the 83-Standard was beneficial as it allows the returned true/false value to be used as a mask for AND.