MOS Technology 6507

The 6507 is an 8-bit microprocessor from MOS Technology, Inc.

It is essentially a 6502 chip in a smaller, cheaper 28-pin package. To do this, A15 to A13 and some other signals such as the interrupt lines are not accessible. As a result, it can only address 8 KB of memory, which for some applications at the time (1975) was acceptable and not overly restrictive. The entire 65xx CPU family was originally conceived as a line of very low-cost microprocessors for small-scale embedded systems, not general-purpose computers and certainly not interactive personal computers (which did not generally exist yet).

The 6507 and 6502 chips use the same underlying silicon layers, and differ only in the final metallisation layer. This ties the interrupt lines to their inactive level so that they are not vulnerable to generating spurious interrupts from noise. The first three digits of the chip identifier are part of the silicon layers, and the final digit is in the metallisation layer. Micro-photography of the 6502 and 6507 shows this difference.

The 6507 was only widely used in two applications: the best-selling Atari 2600 video game console and the Atari 8-bit family floppy disk controllers for the 810 and 1050 drives. In the 2600, the system was further limited by the design of the ROM cartridge slot, which allowed for only 4 KB of external memory to be addressed (the other 4 KB was reserved for the internal RAM and I/O chips, using a minimal-cost partial decoding technique that causes the RAM and peripheral device registers to be mirrored throughout the 4 KB).

Most other machines, notably home computers based on the 650x architecture, used either the standard 6502 or extended, rather than cut down, versions of it, in order to allow for more memory.

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