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Table of Contents
Memory Map
| Addr | Use |
|---|---|
| $0000 - $1FFF | General purpose RAM |
| $2000-$2007 | System control registers |
| $2008 - $2009 | Gamepads |
| $2800 - $280F | Versatile Interface Adapter (GPIOs, Timers) |
| $3000 - $3FFF | Audio RAM |
| $4000 - $7FFF | Framebuffer, Sprite RAM, Blitter registers |
| $8000 - $FFFF | Cartridge slot |
Being a 6502-based system, the GameTank can read at, write to, or execute code from 65,536 different locations during a given cycle. The whole collection of these locations is called the “address space”, where “address” refers to the numeric ID of a given location. These locations are also often referred to as “memory” even though not all of the addresses point to a device you can store data to read back later.
In this context, dollar signs denote that a base-16, or hexadecimal, value is being presented.
General Purpose RAM
The lowest 8,196 addresses provide general-purpose data storage. Data stored in this range will only be read and written by the main processor, without side effects.
Of special note within this section are the ranges $0000-$00FF, and $0100-$01FF, called the Zero Page and the Stack respectively. Addresses in the Zero Page can be accessed more quickly by special opcodes, and addresses in the stack are used by certain operations such as calling and returning from subroutines.
Although the memory map only dedicates 8K to this section, there are actually 32 kilobytes of general purpose RAM that can be used. Four different banks of RAM can be selected using the Banking Register.
System Control Registers
These locations are write-only, meaning that any attempts to read from these locations will essentially return a random value. On the physical hardware the data bus will have no component imparting any particular voltage, so any values returned will come from uncertain forces like radio static or the whims of inscrutable unseen entities.
Addresses $2000, $2001, and $2006 are used for controlling the Audio Coprocessor.
Banking Register
Address $2005 is the Banking Register, which determines which RAM bank is in use, which Sprite RAM bank is in use, and how the Blitter behaves when a draw operation crosses the border of the screen.
| Bitmask | Use |
|---|---|
| 00000111 | Select the active Sprite RAM page |
| 00001000 | Select which framebuffer to read/write/blit |
| 00010000 | Clip blits on the left/right screen edges |
| 00100000 | Clip blits on the top/bottom screen edges |
| 11000000 | Select general purpose RAM page |
When a flag for clipping in a direction is set, blits to coordinates 128-255 will have no effect. When the flag is not set, blits to those coordinates will wrap to the 0-127 range.
Video/Blitter Flags AKA DMA Flags
These flags at $2007 configure the blitter and the behavior of video memory.
| Bitmask | Name | Use |
|---|---|---|
| 00000001 | DMA_ENABLE | Enable/disable the Blitter |
| 00000010 | DMA_PAGE_OUT | Select framebuffer page sent to TV |
| 00000100 | DMA_NMI | Enable NMI signal generated by VBlank |
| 00001000 | DMA_COLORFILL_ENABLE | Use solid colors for blits instead of sprites |
| 00010000 | DMA_GCARRY | Set 0 to repeat 16×16 tiles on blit draws |
| 00100000 | DMA_CPU_TO_VRAM | 0 means CPU accesses Sprite RAM1 means the CPU access the framebuffer |
| 01000000 | DMA_IRQ | Enable IRQ signal when blits finish |
| 10000000 | DMA_OPAQUE | Set 1 to disable transparency |
Gamepads
The front gamepad ports can be read at $2008 and $2009. Writing to these addresses will not do anything. A byte read from one of these locations will return 6 binary button states from the respective controller. Consecutive reads to the same port will toggle the “select” line on the controller port, causing the controller to change which six button states are being returned. More on reading gamepads can be found in its own article.
Audio RAM
Audio RAM is shared between the main system bus and the Audio Coprocessor. From the main processor's perspective this memory sits at $3000-$3FFF. See the dedicated page for more audio specific info.
Video RAM
Video RAM is accessible from $4000-$7FFF and maps to either the Framebuffer, Sprite RAM, or the Blitter Parameters.