Assignment 4, CDA4101, Summer A 1998

Due Thursday 6/11 at the start of class.

1. (1 point) Explain how the SMASK register is used in the microprogram.
2. Suppose that the ROM that holds the microprogram is 512 words instead of 256. This means that an address into the ROM would need 9 bits.
   • (2 point) How can the microinstruction layout of Figure 4-9 be changed to include a 9 bit address field, but without losing any of the current functionality of the instruction and without increasing the size of the microinstruction beyond 32 bits?
   • (1 point) What change would have to be made to the hardware to implement the above modification?
3. The design of the hardware of Figure 4-10 has an N bit and a Z bit. Suppose you are to add the O bit and V bit from the previous assignment.
   • (1 point) How would you implement the O bit (unsigned overflow) in hardware?
   • (1 point) How would you implement the V bit (signed overflow) in hardware? (The V bit is set when the carry into the highest bit is different from the carry out of the highest bit of the ALU)
   • (2 point) Suppose that we added two new microinstructions JMPO and JMPV. JMPO would cause a conditional jump if the O bit was set, and JMPV would cause a conditional jump if the V bit was set. This would require three bits for the COND field of the microinstruction. Devise appropriate names for these three bits and give me the revised boolean statement for the Mmux signal. (The boolean statement for the previous Mmux signal is on page 178).
4. The following two macroinstructions are to be added to the Mac-1 language of Figure 4-14:

   ANDD

   AND direct, ac := ac AND m[x]

   ANDL

   AND local, ac := ac AND m[sp + y]

   You are to modify the machine instructions to include these new instructions. You decide what the operation code for each should be. You will change (and move one of???) the local instructions (LODL, ADDL, STOL, SUBL) to accommodate these new instructions, but you must follow these rules:

   • x will be 12 bits from the machine instruction.
   • y will be 8 bits from the machine instruction.
   • All machine instructions will be 16 bits long.
   • All local instructions (LODL, STOL, ADDL, SUBL, ANDL) will be limited to adding an 8 bit number to the stack pointer, instead of 12 as it is designed in the book.

   Give me the following for this new architecture

   • (3 points) Give me a list of all new and modified conventional machine code instructions. Give me a description of each of these. Indicate any necessary changes to hardware.
   • (3 points) Show me the new and modified microcode for these instructions. Include the line number for modified instructions, add new instructions after line 78 in Figure 4-16. If you need to add instructions between two old instructions, for example between 17 and 18, then label them 17a, 17b, 17c, etc.
   • Do not rewrite the entire microprogram. Just give me the changes and how they would fit into the microprogam. Be sure to add any new decoding instructions, too.
     
5. (4 points) Draw the PLA for figure 4-19. Assume you have a 4-input, 13-output PLA. The input lines are for the four bits of the op code, the 13 output lines are the labels at the top of the figure. Notice that only 15 combinations of the inputs are needed. So the PLA should have four input lines, 15 AND gates and 13 OR gates. Draw the PLA, it will be easier than drawing all the circuits. Only draw the input lines that you need.
6. (2 points) What is the difference in size for Mic-2 (pages 200-201) if a nanostore is used? Include the size of the new control store and the nanostore in your calculation.