User:Kyle8706/sandbox

The 6504 is an 8-bit microprocessor from MOS Technology, Inc. It is a version of their 6502 packaged in a 28-pin DIP, which makes it cheaper to package and integrate in systems. The reduction in pin count is achieved by reducing the address bus from 16 bits to 13 (limiting memory from 64 kB to only 8 kB), and removing a number of other pins only needed in certain applications.

The 6504 was only widely used in the Commodore 1526 printer also known as the MPS-802.

Pin configuration
The 6507 uses a 28-pin configuration, with 13 address pins and 8 data pins. The seven remaining pins are used for power, the CPU timing clock, to reset the CPU, to request bus wait states (the RDY pin), and for read/write commands to memory (or MMIO devices) from the CPU. There is no IRQ or NMI pin on the processor.

The RDY pin is not included on all other 28-pin cut-down versions of the 6502. Within the Atari 2600, RDY is used to synchronise the CPU to the television video frame. This function is essential for the 'racing the beam' method used by the 6502 and Atari Television Interface Adaptor chip to generate the television video signal. In response to a specific register access, the TIA will assert RDY to extend that bus cycle until the end of the current video scan line.