6502 Basics on Programming the Atari VCS in Assembly

Registers & Status Flags

Mon, 01 Jan 0001 00:00:00 +0000

Registers & Status Flags#

The 6502 has astonishingly few places to keep data — three 8-bit working registers, and that scarcity shapes how every program is written. You can’t spread work across a bank of registers the way you would on a bigger CPU; values are constantly shuttled between these three and memory.

Register	Width	Role
A (accumulator)	8-bit	The workhorse. The only register that does arithmetic and logic (`ADC`, `SBC`, `AND`, `ORA`, `EOR`). Almost all data flows through it.
X	8-bit	Index/counter. Used to offset addresses (`LDA table,X`) and to count loops. Also the bridge to the stack pointer (`TXS`/`TSX`).
Y	8-bit	The other index/counter. Similar to X, with its own indexed modes.
S (stack pointer)	8-bit	Points at the top of the stack.
PC (program counter)	16-bit	Address of the next instruction to execute.
P (status)	8-bit	The processor flags — see below.

A, X, and Y are not interchangeable: only A computes, only X and Y index, and each has instructions the others lack. Choosing which value lives in which register is a real part of writing tight 6502 code.

Addressing Modes

Mon, 01 Jan 0001 00:00:00 +0000

Addressing Modes#

An instruction like LDA doesn’t just load — it loads from somewhere, and the addressing mode is how that somewhere is specified. The same LDA comes in several forms, and they differ in both what they can reach and how many cycles they cost — which, on the VCS, is the part you care about most. Reading 6502 Assembly introduced the #/$/,X notation; here is what each mode actually does.

The Instruction Set

Mon, 01 Jan 0001 00:00:00 +0000

The Instruction Set#

The 6502’s instruction set is small — about 56 mnemonics — and you can write whole games with maybe two dozen of them. This page is a grouped tour, not an exhaustive datasheet: enough to recognize what you read and reach for what you need. Cycle counts live in Cycles & Timing; flag effects are in Registers & Status Flags.

Moving data#

The most common instructions by far — nothing computes until data is in a register.

Cycles & Timing

Mon, 01 Jan 0001 00:00:00 +0000

Cycles & Timing#

On most computers an instruction’s cycle count is trivia. On the VCS it is the central fact of the craft: every visible scanline is 76 CPU cycles, and a kernel that needs 77 doesn’t run slow — it tears the picture. This page is the reference for counting them.

A “machine cycle” is one tick of the 6507’s ~1.19 MHz clock; the TIA runs at three times that, so one CPU cycle = three color clocks (three pixels). You budget in CPU cycles and the picture pays out in pixels.

The Stack & Subroutines

Mon, 01 Jan 0001 00:00:00 +0000

The Stack & Subroutines#

The stack is the 6502’s scratch area for return addresses and saved values, and subroutines (JSR/RTS) are built on it. On most 6502 systems it’s an afterthought. On the VCS it deserves real care, because the stack and your variables share the same 128 bytes of RAM — and can run into each other.

How the stack works#

The stack pointer S is an 8-bit offset into a fixed region, and it grows downward: