601.229 (F19): 6502 review problems

This page has 6502 review problems to use studying for the midterm exam.

You can refer to the 6502 Instruction Set reference for details on the various 6502 instructions. The lecture slides are also a useful reference.

Problem 1

Note: this is a somewhat challenging problem.

Write a 6502 subroutine called reverse_bits which reverses the bits in the A register, returning its result in the A register. Reversing bits means that the value of bit 0 becomes bit 7, bit 1 becomes bit 6, bit 2 becomes bit 5, etc. (Think of the bits of A as an array of one-bit values, and reverse the contents of the array.)

It should be called using code like

LDA #value
JSR reverse_bits
; bits in A are now reversed

Here are some example values to try (to substitute for value in the calling code above):

Value	Binary	Value (reversed)	Binary (reversed)
$5a	01011010	$5a	01011010
$68	01101000	$16	00010110
$98	10011000	$19	00011001
$21	00100001	$84	10000100
$c3	11000011	$c3	11000011
$fb	11111011	$df	11011111
$be	10111110	$7d	01111101

Hints: You may want to use memory locations for temp variables. You may use the X and Y registers freely.

Test your function using Easy 6502. You can use calling code as shown above. Be sure to add a BRK instruction after the calling code (i.e., after the JSR instruction) to avoid re-entering your reverse_bits function.

Problem 2

Write a 6502 subroutine called times3. It should modify the value of the A register so that it is three times its original value.

Example calling code:

LDA #value
JSR times3
; value of A is now 3×value

For example, if you set A to $11 (decimal 17), after calling times3 A should be $33 (decimal 51).