How to make smooth horizontal / vertical movement in 6502 assembly (NES)?

Question

I'm currently working in a demo of a platformer in 6502 assembly for a college project, and I'm not understanding how I could implement smooth horizontal/vertical movement and acceleration in this project. Basically, every movement consists in a slight different version of the following code:

READ_RIGHT:
  LDA JOYPAD1
  AND #%00000001
  BNE DO_RIGHT
  JMP READ_RIGHT_DONE

DO_RIGHT:
  JSR HandleAcceleration
  JSR UpdatePlayerPosition

READ_RIGHT_DONE:

  RTI

HandleAcceleration:
  LDA ACCEL + 1
  CLC
  ADC #$01
  STA ACCEL + 1

  BCC @ON_CARRY

  LDA ACCEL + 0
  CMP MAX_ACCEL
  BCC @NOT_MAX_ACCEL
  LDA MAX_ACCEL
  STA ACCEL

@ON_CARRY:
  INC ACCEL
  INC MOV_FLAG

@NOT_MAX_ACCEL:
  RTS

UpdatePlayerPosition:
  LDA PLAYER_X
  STA $0203
  STA $020B
  TAX
  CLC
  ADC #$08
  STA $0207
  STA $020F
  ADC SPEED
  STA PLAYER_X

  RTS

I think I should use fixed-point arithmetics for this, but every single time I've tried, the program didn't work like it was intended. If someone knows what I should do or what I'm doing wrong, please help.

I've tried applying the following logic to the code:

READ_RIGHT:
  LDA JOYPAD1
  AND #%00000001
  BNE DO_RIGHT
  JMP READ_RIGHT_DONE

DO_RIGHT:
  JSR HandleAcceleration
  JSR UpdatePlayerPosition

READ_RIGHT_DONE:

  RTI

HandleAcceleration:
  LDA ACCEL + 1
  CLC
  ADC #$01
  STA ACCEL + 1

  BCC @ON_CARRY

  LDA ACCEL + 0
  CMP MAX_ACCEL
  BCC @NOT_MAX_ACCEL
  LDA MAX_ACCEL
  STA ACCEL

@ON_CARRY:
  INC ACCEL
  INC MOV_FLAG

@NOT_MAX_ACCEL:
  RTS

UpdatePlayerPosition:
  LDA PLAYER_X
  STA $0203
  STA $020B
  TAX
  CLC
  ADC #$08
  STA $0207
  STA $020F
  ADC SPEED
  STA PLAYER_X

  RTS

Instead of smoothly applying acceleration to the sprite, it only made it go a lot faster instantly and nothing else.

Answer 1

This is a slight guess because I've never done any NES programming, but 6502 is a little endian machine. Traditionally 16 bit values are stored with the least significant byte at the low address and the most significant byte at the high address.

In your code you are adding 1 to the high byte first and then doing some weird stuff I don't fully understand and may well be wrong, as follows:

If ACCEL is a 16 bit value, to add 1 to it I would expect something like:

lda accel
clc
adc #1
sta accel
lda accel+1
adc #0
sta accel+1

Another problem is your code after bcc @on_carry. The code immediately following that will only get executed when the carry is set. And if you are adding 1, the only time carry will be set is if the result of the addition is zero (wrapping round from $ff).

So most of the time you end up incrementing both accel and accel+1.

If accel+1 is zero after the add, you then compare accel to max_accel. cmp clears the carry flag if the accumulator is less than the operand. So cmp followed by bcc will jump if A < max_accel

If accel less than max_accel, you do nothing (by going straight to rts), but if it is more, you set accel to max_accel and then you fall through to increment accel. That doesn't seem right at all.

Answer 2

Your problem is caused by not taking care of vertical retrace. When you don't catch the beam or don't set the IRQ correctly, CPU runs your code continuously. What you need is to run your code at a constant speed/FPS. I'm not an expert on NES hardware but from documentation, $2002 address seems to be the answer for a main loop structure. Try adding;

Main:
    // Wait for vertical retrace here
    lda $2002
    bit 7
    bne Main

    // Do your stuff here
    jmp Main

or by using IRQ, try setting your IRQ enterance parameters correctly taking into account the vertical retrace. Every 6502 based 8-bit machine has a method to catch the beam at the top of the screen. You just need to a do a research on "vertical retrace", "cathing the beam" subjects for NES.