If you are determined to build a TTL 650x core, you can start with a pair of 74S219's (non-inverting 74S189's) in which to accomodate the registers, with the obvious exception of the processor status, which requires it be discretely implemented but shadowed in the register RAM. If you have PCL, PCH, X, Y, A, SP, a flag shadow and an impending operand (to catch the data bus input), that will use up half the space. If you then keep the address bus in a couple of additional locations, so you can get into and out of sequence from the PC, you will, once you've studied the MOS Technology hardware reference, easily see how the device works. Keep in mind that by using two tristate buses you can route data from the tristate RAM to the ALU inputs quite easily. Also, if you consider that the clock phases easily support using the ALU to do the address arithmetic as well as operand arithmetic, you'll see why the chip was so small. If you use tristate multiplexers to accomplish the various shifts and rotates, and use a set of XORs to make an adder-subtractor from a pair of '283's, you'll have most of the workings. The rest is a pair of one-hot sequencers to concatenate the logic paths through the ALU on each clock phase. have fun! Dick ----- Original Message ----- From: "ajp166" <ajp166(a)bellatlantic.net> To: <classiccmp(a)classiccmp.org> Sent: Sunday, May 06, 2001 10:46 AM Subject: Re: How many transistors in the 6502 processor?