CS 208: Computer Organization and Architecture

Implementing the PDP-8, Phase 1: the Datapath

Over the next couple weeks, you will diagram in detail a datapath and the associated control to implement most of the PDP-8 instruction set. This assignment will come in three phases: design a datapath, implement the control for a selection of instructions, and finally, implement the rest of the control and put it all together. Phases 1 and 2 will be done in groups, and Phase 3 will be part of the next takehome exam.

For Phase 1, your job is to draw a datapath (analogous to the datapath from your textbook with which we have been working in class) that will be able to support all the op-code 0, 1, 2, 3, 4, 5, and 7 instructions from the PDP-8 instruction set.

Some details:

• You will work with a group of between 2 and 4 people. We will create the groups during class on Wednesday, Oct 26.
• You will hand in your datapath drawing via e-mail to jondich@carleton.edu by 8:00AM Friday, October 28. The drawing should be submitted as either a .png or a .jpg file.
• Your diagram should show all the input lines and output lines, including clock lines (unlike in the textbook, where the clock lines are not shown to reduce clutter), for each element of the datapath.
• Each input or output line should be either unlabelled (in which case it is assumed to be a 1-bit line) or labelled with a slash on the line and an integer value next to the slash showing the number of bits the line carries.
• Use your judgement to provide understandable labels and abbreviations for the various elements of the datapath. PC for the program counter, Data In for an input line on a memory element, etc.
• Keep in mind that the PDP-8 does not have separate Instruction Memory and Data Memory as in the datapath we have been studying lately.
• Unless you indicate otherwise, I will assume that each of your memory elements (PC, RAM, AC, L, and any other memory you find useful) is composed of falling-edge-triggered D flip-flops. If you want a memory element to have different behavior than that, you need to include that information somewhere on your diagram (with an asterisk and a footnote, for example).
• For now, Control can be in the form of blobs as in the datapath we have been studying. We'll begin to fill in the details of Control in Phase 2.
• It's really important to submit your diagram before class on Friday, since we will build a single datapath for the whole class to use in Phases 2 and 3 during class.

Have fun, and let me know if you have questions.