6502 Assembly Instruction not valid

Question

I'm printing to the screen in 6502 assembly

In the monitor I wrote

            STA $01, y    to store the value at the pointer

When I press enter on this line however It says instruction not valid?

Any ideas...?

Answer 1

The addressing mode you most likely want is indirect indexed

STA ($01), Y

Where eg.

A = '@', Y = 81,   Mem dump:

0001  00
0002  04

And the result would be:

     01234 <-- columns
    +--------
 0  | 
 1  |
 2  | @
    |
rows