The GameTank machine
GameTank is spriteCart's second fantasy-machine profile — a browser-emulated model of the real GameTank console, selected by a cart's machine_id header byte 0x02 (M8 is 0x01). Where M8 aims for classic fantasy-console simplicity, GameTank trades that simplicity for hardware fidelity: a cart-build --emit gtr (or the studio's Export .gtr button) compiles the same cart source into a burnable native ROM. See Building & exporting .gtr ROMs for that half of the story.
What changes vs M8
| Capability | M8 | GameTank |
|---|---|---|
| Color | 16-color fixed indexed palette (3 built-in swatches, per-cart choice) | 256-color fixed HSL-derived lookup table, no per-cart choice — color is picked per-pixel while painting a sheet |
| Sprites | Fixed 8×8 tile sheet (256 tiles), drawn with sprite | 8 sheets of 256×256 8-bit pixels, drawn with the variable-rect blit primitive — see Sprite sheets & blit |
| Audio | 5 fixed-role synth voices (sq0, sq1, triangle, sawtooth, noise) | Hybrid tracker + PCM — synth voices or imported sample instruments; see Samples & sample instruments |
| Cart cap | 32 KB flat (only option) | 2 MB flash-2m by default (or 8 KB micro-8k) |
| Cost model | 1 op = 1 cycle, flat 500,000-op budget | Measured native 6502 cycle costs per op / per draw, charged against a 59,009-cycle budget (55,509 with music) — see Cart-Forth subset's execution-model section for the full breakdown |
Selecting GameTank
The studio's Cart tab has a machine picker. Switching a cart to GameTank changes what the other tabs look like — the Sprites tab becomes a 256×256 sheet editor, Sounds gains a Synth/Sample instrument-kind toggle, and a new Samples tab appears. Starting a New cart on GameTank seeds it with a small blit + d-pad starter (the same shape as the sheets tutorial's example below), giving you a compiling cart to iterate from immediately rather than a blank editor.
Authoring gotchas
Column 127 is the border-color register
On real GameTank hardware, the composite video output's side borders take their color from whichever pixel ends up in framebuffer column 127 each scanline — the emulator replicates this faithfully. Gameplay pixels must never touch column 127 unless you're deliberately coloring the border. Practical rule: clamp every draw call so x + w <= 127 (max painted column 126) — this includes wide overlays drawn at another entity's position (an explosion frame wider than the sprite it replaces is the classic miss) and step-based wall checks (a 2px step means the flag condition is x + w > 125, not 126). Sprites may still slide off the left edge freely — only the right edge feeds the border color.
Only ~100 of 128 lines are visible on a real CRT
Per the GameTank manual, a real CRT overscans the framebuffer's top and bottom edges — only about 100 of the 128 vertical lines are reliably visible. Keep critical HUD elements (score, lives, prompts) inside the central ~100 lines; the browser emulator shows the full 128 lines, so a layout that looks fine on /play can crop on hardware.
Cart RAM: 7,424 B native vs 8,192 B in the browser VM
The browser VM gives every cart the full RAM_BYTES = 8,192 (8 KB) cart-RAM window, bounds-checked on every access. Native .gtr builds are smaller and unchecked: the runtime's cart-RAM window is $0200–$1EFF — 7,424 bytes — and out-of-range reads/writes are not guarded (a deliberate SP2 tradeoff — bounds-checking every memory op would cost real 6502 cycles the hardware doesn't spend, and would diverge from the runs in budget in the browser ⇒ runs on hardware contract). A cart written against the VM's full 8 KB that also targets --emit gtr must stay under 7,424 B, or it will silently corrupt adjacent native runtime state with no error in either environment.
Budget + honest throttling
GameTank's cost model doesn't drop work when a frame runs over budget — it finishes the frame's logic and the runner drops to an honest lower effective framerate instead (with a rate-limited warning), rather than truncating game state mid-execution the way M8's op-budget exhaustion does. Full details, including the M8/GameTank contrast, live in Cart-Forth subset — Memory + execution model.