increment or decrement one of 16, 16-bit registers

Tim Wescott · 2017-05-10T21:42:59+00:00

I've been geeking out on the COSMAC 1802 lately -- it was the first processor that I owned all just for me, and that I wrote programs for (in machine code -- not assembly). One of the features of this chip is that while the usual ALU is 8-bit and centered around memory fetches and the accumulator (which they call the 'D' register), there's a 16 x 16-bit register file. Any one of these registers can be incremented or decremented, either as an explicit instruction or as part of a fetch (basically, you can use any one of them as an index, and you can "fetch and increment"). How would you do this most effectively today? How might it have been done back in the mid 1970's when RCA made the chip? Would it make a difference if you were working with a CPLD, FPGA, or some ASIC where you were determined to minimize chip area? I'm assuming that the original had one selectable increment/decrement unit that wrote back numbers to the registers, but I could see them implementing each register as a loadable counter -- I just don't have a good idea of what might use the least real estate. Thanks. -- Tim Wescott Wescott Design Services I'm looking for work -- see my website!

May 10, 2017 22 Replies

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Allan Herriman 9 years ago

Motorola's MC6809 was available in both clocking varieties - The 'E' suffix part number was the one with the external clock generator and two quadrature clock input pins (called E and Q). The non-'E' suffix part number had one clock input pin (EXTAL) and divided by four internally. E and Q were outputs in this case. A pin (MRDY) was available to freeze the divide by four counter to insert wait states.

It had an 8 bit ALU. 16 bit operations took two cycles, and the 8 x 8 multiply took 8 cycles.

I vaguely recall wire wrapping one of these as a hobby project in the early to mid '80s.

Regards, Allan

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rickman 9 years ago

I vaguely recall wire wrapping!

Rick C

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Tim Wescott 9 years ago

Found the patent for the 1801; it shows the increment unit as separate from the registers, and separate from the ALU. I would assume that if it's to be binding, the patent needs to reflect what's actually there, at least in the large:

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www.wescottdesign.com

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