A Street of Numbered Boxes

A byte in a register is a byte you're holding in your hand. But there are only three registers, and you can't put a whole game in your hands. So where do bytes live the rest of the time?

In memory. Picture a street of numbered boxes, each with an address — just a number, like a house number — holding exactly one byte. You write a byte into a free box and read it back whenever you like. You met this in BASIC through a side door — POKE to write a box, PEEK to read one. Here it's your main tool, and it's two ordinary instructions.

But here's the first place the NES surprises a C64 programmer: its street is short. The 2A03 can name 65,536 addresses, like any 6502 — but the NES only wires up 2 KB of actual RAM, the boxes from $0000 to $07FF. That's all the free workspace you get. The rest of the address range isn't yours: $8000 and up is the cartridge ROM (you can read your program there, but you can't store into it), and the ranges around $2000 and $4000 are hardware — the PPU and the sound and controller chips. Write there and you change the machine, not memory. Two kilobytes sounds tiny, and it is; NES games learn to be frugal.

What you'll see by the end

A green screen — value $2a. But as in Unit 2, the colour isn't the point: this value went out into memory and came back. We stored it at a box, wiped the register clean, then fetched it from the box again. The screen is the proof; the real evidence is in the memory view — watch box $0300 fill with $2a.

Store and read: two mirror-image instructions

You already know lda — earlier it loaded a value (lda #$2a). Here it loads from a box (lda $0300). The difference is that little #: #$2a means the number $2a; plain $0300 means the box at address $0300. Forget the # and you load the contents of address $2a instead of the number — a mix-up worth meeting now, because it's the most common one there is.

It's the same round trip you ran for registers in Unit 2 — but the value travels out to a memory box and back, instead of into X:

The program

Same sealed harness; the YOUR CODE block is the round trip:

; ============================================================================
; Meet the Machine (NES) - Unit 4: A Street of Numbered Boxes
;
; Same sealed harness - it shows whatever colour you leave in A. This time the
; value takes a round trip out to a box in memory and back, to prove the box
; held it. The NES has only 2 KB of these boxes: $0000 to $07FF.
; ============================================================================

.segment "HEADER"
    .byte "NES", $1a
    .byte 2
    .byte 1
    .byte $00, $00

.segment "CODE"

reset:
    sei
    cld
    ldx #$40
    stx $4017
    ldx #$ff
    txs
    inx
    stx $2000
    stx $2001
    stx $4010
warm1:
    bit $2002
    bpl warm1
warm2:
    bit $2002
    bpl warm2

    ; ----------------------------------------------------------- YOUR CODE START
    lda #$2a                ; A = $2a (a green)
    sta $0300               ; store A into the box at $0300
    lda #$00                ; wipe A clean
    lda $0300               ; read the box back into A
    ; ------------------------------------------------------------- YOUR CODE END

    sta $00
    bit $2002
    lda #$3f
    sta $2006
    lda #$00
    sta $2006
    lda $00
    sta $2007

forever:
    jmp forever

nmi:
    rti
irq:
    rti

.segment "VECTORS"
    .word nmi
    .word reset
    .word irq

.segment "CHARS"
    .byte $00,$00,$00,$00,$00,$00,$00,$00
    .byte $00,$00,$00,$00,$00,$00,$00,$00
    .byte $ff,$ff,$ff,$ff,$ff,$ff,$ff,$ff
    .byte $00,$00,$00,$00,$00,$00,$00,$00
    .res 8192 - 32, $00

We wipe A to 0 between storing and reading, so the green screen can only mean the $2a came back from memory. sta $0300 is your POKE; lda $0300 is your PEEK.

Assemble and run

ca65 boxes.asm -o boxes.o && ld65 -C nes.cfg boxes.o -o boxes.nes

Load it with the memory view open, and watch box $0300 take the $2a before the screen turns green.

Try this: any free box will do

Change both $0300s to another free RAM address — say $0310. Same green screen. Nothing is special about $0300; any box from $0000 to $07FF holds your byte just as well. (Steer clear of three areas: $0000, which the harness borrows as scratch; $0200, which games reserve for sprites; and anything from $2000 up, where the hardware and the cartridge live — that's not free RAM.)

Try this: store past the end of RAM

Change the store to sta $4000 — an address the NES can name, but one that belongs to the sound hardware, not RAM. Assemble and run. The screen does not come up green: the $2a never lands in a box you can read back, because $4000 isn't memory — it's a register on the APU, and writing there does something else entirely. The lesson: only $0000–$07FF behaves like the street of boxes. Put $0300 back.

If it doesn't work

• The screen is black, not green. The lda $0300 read is missing or mistyped, so A was still 0 from the wipe. Check there's no # on it — lda $0300, not lda #$0300.
• ca65 complains about the store. Check it's sta $0300 (no #). With the # it's not a valid store at all.
• Nothing in the memory view changes. Make sure you're looking at $0300 in the 2 KB RAM window, not somewhere up in the cartridge range.

What you've learnt

Memory is a street of numbered boxes, each holding one byte: sta addr stores A's byte at an address, lda addr reads it back, and the # is what separates a number from the box at that number. On the NES only $0000–$07FF is real RAM — a frugal 2 KB.

What's next

Some addresses aren't quiet boxes at all — write to the right ones and you change what's on the screen. But on the NES the screen doesn't sit in this street the way it does on a home computer. Next — The Screen Is Behind a Window — we finally open up that harness line that's been painting your colour, and meet the PPU.