Why this NES 6502 assembly code doesn´t work when moved to a scoped proc?

Question

I have a snippet that clears memory before initializing a game in NES 6502 assembly. When I leave the code inside the reset proc like so, it works:

.proc reset
  SEI
  CLD

  LDX #0
  ClearRAM:
    STA $000,x
    STA $100,x
    STA $200,x
    STA $300,x
    STA $400,x
    STA $500,x
    STA $600,x
    STA $700,x
    INX
    BNE ClearRAM
.endproc

However, if I try to move this ClearRAM snippet inside a scoped proc:

.scope Memory
  .proc clear
    LDX #0
    ClearRAM:
      STA $000,x
      STA $100,x
      STA $200,x
      STA $300,x
      STA $400,x
      STA $500,x
      STA $600,x
      STA $700,x
      INX
      BNE ClearRAM
    RTS
  .endproc
.endscope

And then load it like so:

.proc reset
  SEI
  CLD

  JSR Memory::clear
.endproc

It stops working. It's like it loops forever in the ClearRAM loop.

Any idea what I am doing wrong?

Thank you!

Answer 1

Your code clears the first 2K of RAM memory. (I don't know if the NES has more than 2K of memory.)

Because the stack is always located in the first 1K of memory on systems using a 6502 CPU (to be more precise: in the range 100h...1FFh), your program also clears the stack.

The JSR saves the address where the program flow shall continue after the RTS to the stack and the RTS instruction reads that address from the stack.

If you erase the content of the stack, RTS will read some bad value from the stack and jump to that address.

If the A register in your program contained the value 12h, the RTS instruction will read 1212h from the stack and program execution will continue at address 1212h (after the RTS instruction) instead of the instruction following the JSR.