The Cosmac 1802 is a one chip implementation of a previous two chip (CDP1801 and CDP18101) set produced by RCA. A major advantage over it's competitors was it's CMOS fabrication. Another advantage was it's support for DMA transfers. Perhaps the most famous use of the Cosmac was in the "Elf." This "personal computer" was introduced as a build it yourself series in the magazine Popular Elctronics in August of 1976. There is even an Elf emulator for windows written by Bill Richman and Chris Radek in Sep 1997. It can be found at this location. I do have an alternative site with instruction set cards for this chip and a few others. The site is in the UK at http://www.comlab.ox.ac.uk/archive/cards.html. Apparently, Harris Semiconductor still produces this chip and has complete documentation.
The 1802 can operate at a maximum clock frequency of 6.4 MHz using a +10V power supply. The device does have an on-chip clock generator. The chip uses a multiplexed address bus which increases circuit complexity by requiring a latch to hold the high order byte.
As noted on the image below,
The 1802 allows implied addressing of data memory. This means that all access to data must be indexed by a general register. This is done by either specifying the register in the op code, or using an instruction that makes the X register identify the correct R register. The 1802's program memory is directly addressed. The flexibility of the address control lines allow program and data memory to exist in a shared address space. Branch instructions are two and three bytes long. The two byte instruction uses paged, direct addressing. The second byte replaces the low order byte of the program counter. This allows for jumps within a 256 byte "page."
The 1802 does not have a status register. However, it does have seven flags that can provide status information.
The 1802's I/O i limited to the EF and Q flags described above. Other I/O peripherals would be memory mapped (as opposed to port mapped.)
External logic can assert an interrupt at any time by pulling INT low. When this happens, the CPU will do the following:
From a programming standpoint, DMA is extremely simple. R0 is loaded with the transfer address, external logic then takes over by asserting DMA-IN* or DMA-OUT*. The asterisk indicates an active low signal.
last modified 26OCT04
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