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Good Morning!

 

sta.wx HMP0,X ; 5 19 -

 

 

(.wx)- Is this storing a 16 bit word into an 8 bit register? Is this indexing the accumulator?

If you check the opcode chart you'll see there's 7 different addressing modes for the STA command. Normally we let dasm decide the proper addressing mode and for:

 sta HMP0,x

---

it would pick Zero Page, X because HMP0 is a zero page address. Of crucial importance for 2600 coding, we know that'll take 4 cycles of processing time.

 

Dasm has a feature known as .FORCE (or Force Extension) which allows you to override the compiler and force a specific addressing mode. The code

 sta.wx HMP0,x

---

tells dasm to compile the instruction using the Absolute, X addressing mode, which takes 5 cycles of processing time.

 

It was done because the following command:

 sta RESP0,x

---

needed to finish at exactly cycle 23, but the routine was 1 cycle short on time. There's no version of NOP that takes only 1 cycle, so by forcing a different addressing mode we can make an instruction use an extra cycle.

 

That bit of code could also have been written like this to get the same results:

        sta HMP0,X     ; 4 18 - store fine tuning of X
        sta.wx RESP0,X ; 5 23 - set coarse X position of object
 

Ah... so .wx is forcing a 16 bit address, not a 16 bit word.. And in this case, the page will be zero, your just specifying it to waste time.. Thanks!

Great code, I'm learning a lot

Great code, I'm learning a lot

Thanks!