INTRODUCTION



     V .BYTE 3    ; assign initial value of 3 to byte V
     H .WORD 290  ; assign initial value of 290 to word H

     Anyway, given that H and V have been reserved and have had
some value(s) placed in them, here are the solutions to the problems:

     Problem 1.
        LDA V     ; get the contents of V
        STA 84    ; and store them in ROWCRS
        LDA H     ; then get the first byte of H
        STA 85    ; and store in first byte of COLORS
        LDA H + 1 ; what's this? the second byte of H!
        STA 86    ; into the second byte of COLORS

     Problem 2.
        LDA 84    ; almost, we don't need to comment this...
        STA V     ; it's just problem 1 in reverse!
        LDA 85    ; first byte of COLORS again
        STA H     ; into the least significant byte of H
        LDA 86    ; and also the second byte
        STA H + 1 ; the high order byte of H

     Do you wonder why we didn't try to move both bytes of H at one
time, as we did in BASIC A +, above? Simple: the 6502
microprocessor has no way to move two bytes in a single instruction!
Honest! (And this is probably its biggest failing as a CPU.)
     Of course, if you have a macro assembler, you could write a
macro to perform these operations. Here is an example using one
macro assembler available for the Atari, though all macro assemblers
will operate in at least a similar fashion. First, we define a pair of
macros:

     .MACRO MOVEWORD
     LDA    %1
     STA    %2
     LDA    %1+1
     STA    %2+1
     .ENDM

     .MACRO MOVEBYTE
     LDA    %1
     STA    %2
     .ENDM

Both these macros simply move their first "argument" into their second
"argument" (and we won't define here just what "arguments" are and
how they work--examine a macro assembler manual for more
information). The first macro moves two adjacent bytes (i.e., a
"word"), and the second moves a single byte. And now we can write
our problem code in a much simpler fashion: