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

The **current_slot_ptr** at current_slot_ptr is an iterator that 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.

Two special values in the X position byte (byte 0) serve as **markers**:

Constant	Value	Purpose
SET_MARKER_EMPTY_SLOT	$00	Marks an unused slot (skip during iteration)
SET_MARKER_END_OF_SET	$FF	Marks end of active objects (reset iterator)

When the iterator (current_slot_ptr) encounters SET_MARKER_END_OF_SET, it resets to the beginning of the array via reset_slot_ptr.