rustiNESs — NES Emulator

Program code

NES Console

Controls

CPU status

A

--

X

--

Y

--

PC

----

SP

--

STATUS

--------

FETCHED

--

ADDR_ABS

----

ADDR_REL

----

OPCODE

--

CYCLES

----

PPU status

Pattern Table 0

Pattern Table 1

RAM

What the 6502 is doing

The 6502 is a tiny CPU. It repeatedly: fetches a byte from memory, decodes what it means, then executes it.

Key ideas

RAM is just a big list of bytes (00–FF) at addresses (0000–FFFF).
PC (Program Counter) is "where the next instruction lives."
A is the main working register ("the CPU's hand").
Fetch → Decode → Execute happens over several cycles.

This demo program

A9 42 = LDA #$42 → put 42 into A
8D 20 00 = STA $0020 → write A into RAM at $0020
4C 00 00 = JMP $0000 → set PC back to $0000 (loop)

Result: RAM at $0020 becomes 42 and stays that way.

What to watch

PC highlight shows where the CPU reads next.
OPCODE shows the current instruction byte.
ADDR_ABS shows the computed memory address.
A should become 42 after the first step.