see Bank Switching
In this bankswitching scheme the 2600's 4 KiB cartridge address space is broken into two 2 KiB segments. The last 2K in the cart always points to the last 2K of the ROM image, while the first 2K is selectable. You access
$1FE0 to $1FE6 to select which 2K bank. Note that you cannot select the last 2K of the ROM image into the lower 2K of the cart!
7 x 2 KiB ROM banks switched by write to $1FE0-$1FE6
1 x 2 KiB ROM bank unswitchable
4 x 256 B RAM banks switched by write to $1FE8-$1FEB
Accessing 1FE7 selects 1K of RAM at 1000-17FF instead of ROM!
The 2K of RAM is broken up into two 1K sections. One 1K section is mapped in at 1000-17FF if 1FE7 has been accessed. 1000-13FF is the write port, while 1400-17FF is the read port. The second 1K of RAM appears at 1800-19FF. 1800-18FF is the write port while 1900-19FF is the read port. You select which 256 byte block appears here by accessing 1FE8 to 1FEB.
There are 8K, 12K and 16K variations, with or without RAM.
Examples
16K M-Network bankswitched games.