6502 Basics#

The VCS is driven by a 6507 — a cost-reduced 6502 with a 13-bit address bus (so it can only see 8 KB) and no interrupt pins. Everything you write is 6502 machine code, and the defining constraint is cycles: the CPU runs at ~1.19 MHz and you get roughly 76 machine cycles per scanline. Knowing how many cycles each instruction costs is not an optimization detail here — it is how you keep the picture stable.

This is the programmer’s reference the rest of the book leans on. Reading 6502 Assembly covered the notation; this chapter covers the machine the notation drives — its registers, the ways instructions reach memory, the instruction set itself, and (the part that matters most on the VCS) exactly how many cycles everything costs. For where this CPU sits in the system, see the Architecture CPU page.

• Registers & Status Flags — the three working registers, the stack pointer and program counter, and the status flags branches test.
• Addressing Modes — how an instruction names its operand, why zero page is cheaper, and the page-crossing penalty.
• The Instruction Set — a grouped tour of the ~56 mnemonics, and the few that do most of the work.
• Cycles & Timing — the cycle cost of every common instruction, and WSYNC. The reference Kernel Techniques builds on.
• The Stack & Subroutines — JSR/RTS, push and pull, and the VCS quirk that the stack shares your 128 bytes of RAM.

If you are new to the 6502, the registers and addressing modes are worth internalizing before the TIA chapter — almost every drawing technique is expressed as tightly cycle-counted loads and stores.