[teh_klev]

Not to belittle the article, because it's definitely interesting. But as an ex-BBC 6502 programmer, my nitpick here would be that the title should really be named "Dirty tricks C64 6502 programmers use".

On the beeb we had our own set of tricks specific to the memory layout and ROM of our beloved beige and black machines.

[reaperducer]

title should really be named "Dirty tricks C64 6502 programmers use".

In that case, probably Dirty Tricks 6510 Programmers Use would be even better.

[vidarh]

While that may be technically correct, the programmer-visibile difference between the MOS 6502 and the MOS 6510 is totally incidental in this case - the 6510 has a a built in 6/8-pin IO port (partially used for bank switching the ROMs in the C64). Unless you touch the IO ports, they should behave identically, down to the cycle timings of instructions and the same behavior of undocumented opcodes.

In this case, the real C64 specific tricks are not 6510 specific, but depending on the specific initialization done by the C64 ROM and calling C64 ROM routines.

[tenebrisalietum]

A real "dirty trick" is reading the actual RAM in memory locations 0 and 1 on the 6510 (and not the I/O port values).

[joezydeco]

Yeah the moment they introduced a ROM call to optimize the routine, I kind of lost interest.

[scarface74]

Why? The memory layout of the text page would have also been c64 specific.

[joezydeco]

Because the article was titled "dirty 6502 tricks" and not "dirty C64 tricks".

If the article was more about hidden opcodes or instruction set side effects, that's great. But if the key to shaving 8 bytes out of the routine was calling a 120-byte function that's free in the ROM, there's no real trick to 6502 programming being revealed.

It's the equivalent of a blog post titled "I wrote a web server in 3 lines of code!" and the author calls .start() in some imported framework that contains 250,000 lines of code.

[scarface74]

True. But in his defense, once you want to put text on the screen, you’re already using implementation defined behavior. Like I said in another reply, the character to memory location mapping is different between for instance the C64 and the Apple //e.

The Apple //e would have had a text scrolling function in ROM.

[tom_]

It's a grid of 24 rows (or is it 25?) of 40 bytes, top to bottom, left to right, one byte per char... not exactly uncommon for character mapped displays.

[scarface74]

That wasn’t how the Apple II stored characters. The memory didn’t map contiguously to screen positions where you could calculate the character position using $400+x+y*40.