The viewport is the visible portion of the game world displayed on screen. It contains a fixed-size array of slots that hold active game objects (enemies, fuel depots, etc.) currently visible or about to scroll into view.

The viewport is implemented as the viewport_slots array with 15 slots (46 bytes total: 15 × 3 bytes + 1 end marker byte).

A slot is a 3-byte entry in the viewport array (viewport_slots) that can hold one game object. Each slot has the following structure:

Byte	Contents
0	X position (0-255 pixels)
1	Y position (0-255 pixels, increases as object scrolls down)
2	Object definition byte (type, orientation, activation state)

A slot can be in one of three states:

State	X Value	Description
Empty	$00 (SET_MARKER_EMPTY_SLOT)	Slot is unused, skipped during iteration
Occupied	$01–$FE	Contains a valid game object
End marker	$FF (SET_MARKER_END_OF_SET)	Marks end of active objects; iterator resets to start via reset_slot_ptr

The iterator at current_slot_ptr traverses slots during the main object processing loop (operate_viewport_slots). It advances through all slots (including empty ones), checking each slot's state before processing.

The object definition byte (byte 2 of each slot) encodes the object's type and state using bit fields:

Bit(s)	Meaning
0-2	Object type (OBJECT_* constants: helicopter, ship, tank, etc.)
3	Rock flag (1=rock decoration on land, 0=interactive object)
4	Unused
5	Tank location (1=river bank, 0=bridge)
6	Orientation (1=left-facing, 0=right-facing)
7	Activation (1=active/interactive, 0=spawning/inactive)

Objects spawn with bit 7 clear (inactive). On their first frame in the viewport, operate_viewport_slots sets bit 7 to activate them, enabling movement and collision.