CS 202 - Homework 10 - 11/19/07

Due: Friday, 11/30/07, in class (or by 2pm the latest)

This homework has 3 parts and covers floating point representations and the first half of the Sniac circuit project. You have to do parts 1 and 2 by yourself, but on part 3 you are encouraged to work in groups of two. I only need one submission per group on part 3. On your submissions for each part, please write down how much time it took you.

Part 1: Floating point representations - written problems

1. Tanenbaum, Appendix B, problems 1.c, 1.d, 2.c, 2.d.
2. Martin, Chapter 2, problems 24, 25, 26

Part 2: Floating point representations - C programming

  floattest 0
  floattest 1
  floattest 1.5
  floattest 3.14159265
  floattest -.2
  floattest 3755.8042
  floattest 1.234e-20

Part 3: Sniac II circuit - registers, arithmetic, memory, and busses

Sniac processor organization

A	Accumulator
IC	Instruction counter
MA	Memory address register
MB	Memory buffer
OP	Opcode register
B	B register (for second operand)

These registers are interconnected in the following way (the circles represent tri-state buffers):

Components

1. An 8-bit register with 8 data inputs, 8 data outputs, and control inputs "c" (clock), "ld" (load), and "r" (reset) that uses 8 D-flip-flops to store a byte. Since the entire circuit will use negative edge-triggered synchronous logic, build your D-flip-flops from LogicWorks's "JK Flip Flop". Connect "c" to all clock inputs, and "r" to all (inverted) reset inputs. We'll use an inverted reset signal (r=0 means reset) in the entire circuit.

2. An 8-bit tri-state buffer with non-inverting "enable" input. You can build it from one "Buffer-8 T.S." and one NOT gate.

3. Your 8x8 ROM and 8x8 RAM from HW 7 and 9.

4. An 8-bit adder/subtracter with 16 inputs (A0..7 and B0..7), 8 outputs (S0..7), and two control inputs, "zero_A" and "sub_B". If sub_B=1, the circuit should compute S = A-B. If zero_A=1, it should set A to 0 before adding, i.e., it should simply compute S = B. You can build this circuit from 2 of your 4-bit adder/subtractors from HW 5 plus some extra gates. Or, you can (like my design) use LogicWorks's "Adder-8" together with 2 subcircuits that can set A=0 and invert B.

5. A 5-bit incrementer (to compute IC+1). You can build this using 5 half-adders, or you can simply use LogicWorks's "Incrementer-8 wo/Car" and ignore the upper 3 bits.

Since this is a fairly complex circuit, I'm providing a picture of my own implementation. Your job this week is to implement a circuit like this. Feel free to copy my design, or just use it as a rough guideline, or ignore it altogether. My design could definitely be improved - for example, IC and MA (and the buffers connected to them) should really only be 5 bits wide. There should also be a cleaner way to lay out the various busses. Like my circuit, however, your circuit should have plenty of probes and hex displays so that you can observe the contents of all registers. Also, it is a good idea to keep inputs on the left and outputs on the right when possible.

For this first part of the Sniac project we'll use switches for all control signals. You should test your circuit by setting these switches. Initially, with all switches set to 0 and after pushing the reset button, all displays should show 0, except for the data lines, which should be in "Z" mode. When you set read=1 you should see the contents of memory cell 0 on the data lines. With inc_IC=1 and ld_IC=1, pushing the clock button should cycle IC=00, 01, 02, ..., 1e, 1f, 00. You can test your entire circuit in this manner. In the final homework of the semester you'll add the circuitry to generate these signals automagically.

What to hand in

Have fun!