a Primary Mathematics question

[Boolean]

I have a question about "nes-test-roms\full_palette\full_palette.s".

Code: Select all

  ; Delay 4739 cycles, well into frame
  ldx #4   
  ldy #176 
: dey     
  bne :-  
  dex     
  bne :-

The author says 4739 cycles delayed. What I calculated is as following:
; Delay 4739 cycles, well into frame
ldx #4
ldy #176
: dey
bne :-
dex
bne :-

result = 2 + 2 +
(176 x 5 - 1) + 3 x (255 x 5 - 1) +
(4 x 5 - 1)
= 4724

Why my answer is wrong?

[tokumaru]

I didn't do the math, but I believe your problem is in the 255, which should be 256.

If you're counting 176 iterations for a counter loaded with 176, why are you counting only 255 iterations for a counter loaded with 0? In this case, 0 works the same as 256.

[koitsu]

tokumaru is correct -- the formula should be 2 + 2 + (176*5-1) + 3 * (256*5-1) + (4*5-1) which equals 4739.

[Boolean]

Thank you. You two are right.
At the same time, I find
it is good to use a familiar programming language to emulate 6502 codes.

Here comes my solution.

[nIghtorius]

Why not

Code: Select all

/*
; Delay 4739 cycles, well into frame
  ldx #4   
  ldy #176 
: dey     
  bne :-  
  dex     
  bne :-
*/

#include <iostream>
using namespace std;

int main () {
	int	Cycles = 0;
	int	X = 4, Y = 176;
	Cycles += 4;

	while (X != 0) {				
		while (Y != 0) {
			// dey
			Y--;
			Y &= 0xFF;
			Cycles += 2;	
			// bne :-
			if (Y != 0) Cycles++;
		}
		// dex
		X--;
		X &= 0xFF;
		Cycles += 2;
		if (X != 0) Cycles++;
		// bne :-				
	}
	cout << "Cycles is " << Cycles << endl;
	return 0;
}

??

I find goto's in C++ weird.

[Boolean]

I find goto's in C++ weird.

Because mine is closer to original 6502 code and is more easier to understand.
Yours is more reasonable in real life, but it's farther from assembly languages.
The keyword "goto" is certainly not recommended, but it is useful in some cases.