Since you already know how to create loops using IF statements, use ROM calls and make your own subroutines, learning how to use GET_KEY should be a snap. Getting right to the point, here's the basic read key procedure:

GetLoop:
     call GET_KEY
     or   A
     jr   z, GetLoop
     ret

The GET_KEY routine itself returns in A the keycode of the key being pressed, or 0 if no key is being pressed. OR A causes the Zero flag to be updated, which is used to check whether a key was pressed. OR A is shorthand for OR A, A, and anything OR'ed with itself is still itself, so the value in A doesn't change. However, OR A does update the Flags register. JR Z checks the updated Flag register to see if A is zero. If A is zero, then no key was pressed. In that case we jump back to the top of the loop and try again. If A is not zero, we read a key, so we end the procedure and return to the rest of the program, leaving the keycode stored in A.

The keycodes returned by GET_KEY are different than the ones returned by the TI-BASIC procedure getKy. You may want to download the table of GET_KEY keycodes and print it out for a reference. The sample program you will write and assemble in the next two lessons will return scancodes of pressed keys using this GetLoop subroutine.

In the next lesson you will write and assemble a usable program. Choose the lesson that matches your platform.

Using TASM to assemble programs written on an IBM PC.

Using CAZ to assemble programs written on a Mac.

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This page created 3-7-96 by Greg Parker.
Last modified: Tue Aug 19 22:50:59 PDT 1997