----- Original Message -----
From: Wade Brown
To: stella@xxxxxxxxxxx
Sent: Thursday, August 09, 2001 10:18 PM
Subject: Re: [stella] Help (Timing problems?)
> Maybe someone out there has a link to a better opcode ref - If so I (and
the list, no doubt) would sure appreciate hearing about it.
I´ve download from I-don´t-remember-where a fairly complete documentation
for the SY6500, which for what I know, is equivalent to the 6502. I had been
using it for a while without any problem (be sure to ignore the SY65C02
part, it
has newer features which aren´t available on the 6502). If you can´t find it
on the web, let me know and I will email it to you.
You can also try with the assembly language chapter from the C64 manual, but
I don´t know if it can be found online, anyway it won´t tell you much
more than the SY6500 documentation.
>> bcc .inXBounds
>> lda #RIGHT_BOUND
>> .byte $2c ; opcode for bit.w, this hides the next
instruction <-- I don't understand? (Wade)
>>.outOfLeftBound:
>> lda #LEFT_BOUND
>> sta Player1X
>>.inXBounds:
I will try to explain this, but I know my english will fail me, sorry.
.byte $2c is a directive for the compiler, not an opcode, so it won´t be
interpreted as one, and simply $2C will be written to the binary in that
position.
When this code is executed, $2C will be fetched, which is the value for the
opcode BIT in absolute addressing mode.
That mode needs two bytes of data to determine a memory location, so, the
next instruction lda #LEFT_BOUND (since it´s two bytes long), will be
considered data, instead of being executed. Notice that when bcc
outOfLeftBound: is taken, lda #LEFT_BOUND will be executed.
The purpose of this, is to save ROM space, because just one more byte allows
you to store the left or the right bound on Player1X .
Bye,
Nicolás.
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