Lab 9 - The complete computer


To add machine code instructions to the computer. We need to modify the current hardware to allow us to fetch instructions, and then map to the correct microcode routine to execute that instruction. To do this we need to:

  1. A OPCODE mapper. This is logic to take an opcode, and causes the microsequencer to jump to the microcode for this instruction.
  2. Add a bit to the microcontrol word called "Fetch". This tells the computer to read the next instruction pointed to by the PC and send the OPCODE bits to the microsequencer for decoding.
  3. Add a PC. Rather add a new register we can simple re-purpose the B register as a PC. This gives the added benefit that we can manipulated the PC in the same way the other registers are manipulated.
  4. Clear the PC on reset. We do this by asynchronous clear to our register.
  5. Allow the memory address to come from either the MAR or PC. The address with come from the MAR unless a fetch is happening.
  6. Store the N and Z bits in flop-flops. We need to do this so that a branch instruction can access the previous instruction's results.
  7. Add a way to get only the lower 8 bits of an instruction, which is often either an address or data. We will do this by adding an alternate path for putting memory output on the A bus.
  8. Finally we need to add the microcode We will only start this in this lab, and continue it in future labs.
Note: The "C" register from the prvious lab has been renamed "B" for a bit more consistancy.



This is a pivotal lab in the course of building your complete computer. It is very important that you complete it this week in order to move on to the next lab. It is fairly complex, but I will be spending time in class talking about it to get things going. We will first modify the computer to allow us to fetch instructions from memory, and map those instructions to a place in the microcode where it can be executed.

The system will operate using the following flowchart.


The goal of the system is to implement the following instructions.


These are the ISA level instructions we will be supporting. The instruction format (in general) is:


Where add or n below is the value of bits [0-7] in the nstruction, and the opcode is in bits [8-11].

Opcode Mnemonic Meaning Description
0 AND add ACC <- ACC ^ mem( add) AND memory - AND memory location "add" with the ACC
1 ADD add ACC <- ACC + mem( add ) Add memory - add memory location "add" to ACC
2 LOAD add ACC <- mem( add ) Load memory - load memory location "add" to ACC
3 STORE add Mem( add ) <- ACC Store memory - Store the ACC into the memory location "add"
4 SUB add ACC <- ACC - mem( add) Subtract memory - subtract memory location "add" from the ACC
5 MUL add ACC <-ACC * mem(add) Multiply the ACC by a memory location
6 DIV add ACC <- ACC/mem(add), B <-remainder Divide instruction, uses B for remainder
7 BR add Branch to add Branch to address add
8 BEQ add Branch to add if Z=1 Branch to address add if the result of the last computation was 0
9 BLT add Branch to add if N=1 Branch to address add if the result of the last computation was negative
F00 CLRA Clear ACC Clear Acc
F01 NOT Not ACC Not the ACC
F02 NEG Negate ACC Negate the ACC
F03 INC ACC = ACC + 1 Increment the ACC
F04 DEC ACC = ACC - 1 Decrement the ACC
F05 SWAP ACC <=> B Swap the ACC with the B register
F0E NOP Do Nothing Do nothing
F0F HALT Stop execution halt computer
Instruction Format

These instructions are in two formats.

  1. Type 1 Instructions - these are instructions with an explicit operand in the lower 8 bits (ADD, LOAD, STORE, ....). There can only be 16 of these types of instructions, because there are only 4 bits available for the opcode.
    so, for example, the instruction 0x103 will Add memory location 3 to the accumulator and 0x405 will load the value 5 into the accumulator.
  2. Type 2 Instructions - these are instructions with no explicit operand in the lowver 8 bits (CLRA, NOT, NEG, INC, ....). We can user the lower bits to specify more instructions because they are not holdoing data. We will use the opcode=1111 (0xF) for this group of instructions. For the time being we will only use bits 0-3, signified as x's below.
    so, for example, the instruction 0xF02 will negate the accumulator.


Step 1 - Build an op-code mapper

An opcode mapper takes each op-code and maps it to a place in ucode where the instructions to implement that instruction are contained. There are actually two type of instructions:

  1. What we need to do is to map these instructions to a place in memory. What we can do with group 1 above is simply form a microcode address as the opcode followed by three 0's. If we also set bit 7 to zero, we end up with something we see with Type one instructions below. This means that the each instruction will map microcode memory location opcode*8. This will give up to 8 micro instructions for each ISA instruction.
For type 2 instructions we will set bit seven to 1, and use bits 0-3 from the instruction as bits 3-6, just as we did in type 1 with the opcode. This will map the instructions 8 words apart starting from microcode memory address 0x80.

Below we see the two address mapping rules:


One exception to the rule - the ADDI instruction. Instead of mapping to zero, it will map to microcode memory location 1! Why? Because we one to store the fetch instruction in memory location 0. This will greatly reduce the complexity of the computer (as you will see).

Thus the first microcode memory will be something like this:


Type 1 Instructions
Opcode Hex Mnemonic Meaning Address
0000xxxxxxxx 0xx AND add ACC <- ACC ^ mem( add) 0x01
0001aaaaaaaa 1aa ADD add ACC <- ACC + mem( add ) 0x08
0010aaaaaaaa 2aa LOAD add ACC <- mem( add ) 0x10
0011aaaaaaaa 3aa STORE add Mem( add ) <- ACC 0x18
0100dddddddd 4nn SUB add ACC <- ACC - mem( add) 0x20
0101aaaaaaaa 5aa MUL add ACC <-ACC * mem(add) 0x28
0110aaaaaaaa 6aa DIV add ACC <- ACC/mem(add), B <-remainder 0030
0111dddddddd 7dd BR add Branch to Address 0x38
1000aaaaaaaa 8aa BEQ add Branch to Address if Z=1 0x40
1001aaaaaaaa 9aa BLT add Branch to Address if N=1 0x48
1010aaaaaaaa Aaa ??? Future Expansion 0x50
1011xxxxxxxxx Bxx ??? Future Expansion 0x58
1100xxxxxxxxx Cxx ??? Future Expansion 0x60
1101xxxxxxxxx Dxx ??? Future Expansion 0x68
1110xxxxxxxxx Ex ??? Future Expansion 0x70
1111xxxxiiii Fxi see below Zero Op Instructions 0x80+i*8
Type 2 These instructions use bits [0-4] to determine one of 16 instructions. These instructions map to microcode memory address 256+8*[0-3].
Opcode Hex Mnemonic Meaning Address
111100000000 F00 CLR Clear ACC 0x80
111100000001 F01 NOT Not ACC 0x88
111100000010 F02 NEG Negate ACC 0x90
111100000011 F03 INC ACC = ACC + 1 0x98
111100000100 F04 DEC ACC = ACC - 1 0xA0
111100000101 F05 SWAP ACC <=> B 0xA8
111100000101 F06 ??? Future Expansion 0xB0
111100000101 F07 ??? Future Expansion 0xB8
111100000101 F08 ??? Future Expansion 0xC0
111100000101 F09 ??? Future Expansion 0xC8
111100000101 F0A ??? Future Expansion 0xD0
111100000101 F0B ??? Future Expansion 0xD8
111100000101 F0C ??? Future Expansion 0xE0
111100000101 F0D ??? Future Expansion 0xE8
111100000110 F0E NOP Do Nothing 0xF0
111100000111 F0F HALT Stop execution 0xF8

ISA Instruction Mapper

Below is a device to map the 12 bit instrution code to the microcode address as defined above.



  1. The muxiplexor routes either the top four bits (8-11) to the output the bottom for bits (0-3).
  2. The output of the multipler is sent to bits (3-6) of the microcode address.
  3. If the opcode is F (1111), bits (0-3) are send to the microcode address (because it's a type 2 instruction) otherwise bits (8-11) are sent.
  4. In the case where the opcode is 0 (0000) (ADDI), then set to low bit (0) to 1 so that we avoid using memory address 0.
To Do

1.1 Build the circuit above.

1.2 Test it out for several instructions of each type (1 and 2)

1.3 Make sure it works for ADDI (opcode 0)

1.3 Paste a picture of it with a type 1 and a type 2 instruction coming in.

Step 2 - Update the microcode sequencer.

The sequencer from the previous lab can now be extended to include the instruction mapper. As you recall, the previous sequencer either incremented and used the micro-PC for the next address, or used the addess embedded in the microinstruction to determine the next microcode instruciton address. We now have a third option for the next address, the output of the instruction mapper.

What we need to do is to modify the sequencer to integrate the mapper in to also allow the instruction to determine the next mircocode address. This is done below by adding a muliplexer to select between the microcode address field and the instruction mapper. See the diagram below for the implementation. (I have added a probe to the multiplexer output to help with understanding and debugging).


To Do

2.1 - Modify your sequencer to use the mapper.

2.2 Test it out. First make sure the new mapper works in place by adding a "Fetch" micro-instruction (0x80000) to the ROM at location 0, and placing several different OPCODE inputs. See if the sequencer jumps to the right next address. Take a screen shot of at least one successful mapping operation.

2.3. Make sure the mapper still works for the other cases, e.g. for normal stepping though memory one instruciton at a time, and for both conditional and unconditional branches.

Step 3 - Add and test new sequencer to main computer.

The circuit below shows the sequencer finally added to the main computer. This is it. If you get this together you finally have a fully capable system, waiting for it's instruction set to be built in the microcode! Build the system as seen here. There are a few IMPORTANT changes:


  1. Notice the D flopflops connected to the ALU. These are very important. Now that we we are integrating the full system we need to be able to have one machine instruction (that is a branch) use the conditions of the previous instruction to make it's branch decision. For axample, we might load a value into the ACC, and then want to branch if the value is a zero. At the instruction level, one we have moved the data, and are now on to the next instruction, the very act of fatching the next instruction will change the output of the ALU based on this fetch. So we need to be able to "remember" the status one clock cycle back in time.
    The solution is the flipflop. At the end of every CPU cycle, the current output of the Z and N bits are locked into the flopflops, then on the NEXT cycle, the Z and N bits represent the previous state of these bits. So any desision needs to be made exactly one clock cycle after the ALU computes a result.
  2. Another ALU change (sorry). In order for the Z and N bits to be useful, they need to read meaningful data from the output of the ALU. Basically this designe will sense the output of the ALU at the end of each operation, and these values will be available for the next clock cycle. But what about operations that don't involve the ALU (like branch, fetch, NOP). After some thought, I realized that the BEST default behavior is to have the ACC pass through the ALU if nothing else was done with the ALU. So I rearrainged the ALU to have the B input (the ALU) be the default (e.g. when the function bits are 000). Here is a modified ALU, and an actual download of it to speed things up:ALU9.png
    ALU9.circ: The New ALU Circuit
  3. As mentioned above, old C register is renames B, just to make more sense (we now have an ACC and B register.)
  4. We need a PC, that is a way to keep track of the instructions as we fetch them. We can take the B register and turn it into a PC. This requires that the PC now also (like the MAR) can act as a memory address as well. I have thus added a multiplexer to select between the MAR and the new PC as a address input to the memory address bits.
  5. What we need is a new microinstruction bit to select between the MAR and the PC. We call this the "Fetch" bit. We select the "Fetch" micro operation when we wish to use the PC as the address of the next instruction. Thus this Fetch bit (bit 19, or 0x80000) is used as the input to the new multiplexer.
  6. The PC is now a special register. When we turn the computer on, we need to start things up correctly, that is start executing code from some known location to "boot" the computer. This will be memory location zero. Thus we need to clear the PC every time we hit the reset button. So a new type of register has been created with a clear line connected to the reset:
  7. When we fetch an instruction we now have to have a way for grabbing and using the bottom 8-bits of an instruction as an address. The easiest way to achieve this is to use one of the unused outputs on the source decoder to select an alternative output from memory. I have used the 7th pin on the decoder (6, or 110) to select the output from memory that has been routed through a simple device that masks off the top 4 bits, leaving only the lower 8. Below is the insides of this device:
  8. Before there would only two ways we accessed memory two ways, either to write (assert str and sel inputs) or read(assert sel and ld inputs). Now there are four ways:
    • Write to memory (as before)
    • Standard read from memory (as before)
    • When we fetch from memory (a read also)
    • When we read the lower 8-bits) (also a read)
Thus the three or gates below the memory are used to correctly enable the memory inputs for these four conditions. Study them carefully, and then add them
  1. I have added a few probes to the system to show the system state. This is very useful in debugging you system.
  2. I have added an actual clock. A clock outputs a series of pulses, and can be useful when actually running a program. The "clocked" selector lets you decide between either a manually stepped system, and clocked system. The clock must be enabled in the top "Simulate" menu, and the tick frequency can be changed there.

To Do

3.1 Complete the computer as seen above.

3.2 Test the reset operation. The PC should clear on reset.

3.3 Test the sequencer with the mapper in place. Put a fetch (0x80000) in memory location 0. Put an instruction in memory. Try an "ADD" (see the instructions in the table in he next section), that is put 105 in location zero in main memory (ADD's memory location 5 to ACC). Step through the program an verify that as soon as you start, the microcode jumps to the microinstruction in location 0x08 in the ROM. (This is where we will be writing the microcode for the add instruction). Make sure also that the sequencer still increments when not fetching.

3.4 Come up with a strategy for testing the new hardware to get the lower 8 bits of the memory. Describe your test, and show a picture of the results.

3.5 Test the new clock. Explain how you tested it, and how it worked.

Step 4 - Start micro-coding

Below are the current tables for all the features available for the microcoded processor. Review them carefully.

Microcode instructions

Below is the definition for the micro-program control words:

Branch Address Unused Fetch Reset Halt Uncond Branch Z code N Reg Func Cin ALU Func Data Source ACC B PC MAR Mem
31-24 23-20 19 18 17 16 15 14 12-13 11 8-10 5-7 4 3 2 1 0
0xFF000000   0x80000 0x40000 0x20000 0x10000 0x8000 0x4000 0x3000 0x800 0x700 0xE0 0x10 0x8 0x4 0x2 0x1

The bit fields are defined as follows:

ALU Functions

Bits Operation
000 ACC
001 Reg
010 ACC'
011 Reg ^ ACC
100 Reg V ACC
101 Reg + ACC + Cin
110 -Reg
111 ACC-Reg

Data Source

Bits Source
000 Input
001 Memory
010 MAR
011 PC
100 B
101 ACC
110 Memory[0-7]

Reg Func

Bits Func
00 Load
01 Clear
10 Increment
11 Decrement

Now let's consider a very simple test. We will add the microcode for the ADD instruction, and the CLR (clear ACC) instructions. We can use the following advanced microprogramming tool to create the microcode: Microcode generator version 4.

We need to start microprogramming the system. We set location zero to a fetch instruction. The add instruction will map to memory location 0x8, so we put the code for it there. And the clear instruction will map to location 0x80, so we put the code for that there:

Instruction Address Microcode Operation
Fetch 0 00080000 Fetch and dispatch instruction
ADD 8 000001c2 MAR ⇐ Mem(PC)[0-7]
  9 00000530 ACC ⇐ Memory+ACC+Cin
  A 00002004 (PC) ++
  B 00080000 Fetch
CLRA 80 00001010 (ACC) ⇐ 0
  81 00002004 (PC) ++
  82 00080000 Fetch
Halt F8 00020000 Halt

To Do

4.1 Use the program online to generate the code above. Use the save button to save the microcode. Load the microcode into the ROM of the micro-sequencer.

4.2 Create a program:

    ADD 5
    ADD 6

Put this in main memory as:

Address Code Assembler Code
0 F00 Clear ACC
1 105 Add mem(5) to the ACC
2 106 Add mem(6) to the ACC
3 F0F Halt

Put a number in mem(5) and mem(6) to add. Try it out.

To Do

4.1 Get the above program to work

4.2 Show a shot of the final result.

4.3 Writeup: What did you learn? What challenged you? What overall did you think of the lab?

4.4 If your find a spelling or grammar error, and are the first to report you can get 1% extra credit.

4.5 If you find, and propose a fix that works, to a technical issue, your will get 5% extra credit.


Topic attachments
I Attachment Action Size Date Who Comment
Circcirc ALU9.circ manage 11.6 K 2014-11-04 - 16:05 JimSkon The New ALU Circuit
Pngpng ALU9.png manage 29.0 K 2014-11-04 - 16:02 JimSkon  
Pngpng CompleteCPU.png manage 31.7 K 2014-11-02 - 14:07 JimSkon  
Pngpng Lab_8_-_InstFormat1.png manage 3.6 K 2014-10-27 - 04:38 JimSkon  
Pngpng Lab_8_-_InstFormat2.png manage 2.4 K 2014-10-27 - 04:39 JimSkon  
Pngpng Lab_8_-_Map1.png manage 5.9 K 2014-10-27 - 04:43 JimSkon  
Pngpng Lab_8_-_Map2.png manage 6.3 K 2014-10-27 - 04:44 JimSkon  
Pngpng Lab_8_-_micro_flow.png manage 18.8 K 2014-10-27 - 04:00 JimSkon  
Pngpng MemoryControl.png manage 5.1 K 2014-11-02 - 14:07 JimSkon  
Pngpng PC.png manage 5.5 K 2014-11-02 - 14:07 JimSkon  
Pngpng Statusbits.png manage 3.2 K 2014-11-02 - 14:15 JimSkon  
Pngpng instructionmapper.png manage 9.6 K 2014-11-03 - 17:49 JimSkon  
Pngpng lower8.png manage 2.0 K 2014-11-02 - 14:07 JimSkon  
MemMapEXT memMap manage 16.4 K 2014-10-27 - 05:33 JimSkon  
Pngpng memMap.png manage 31.8 K 2014-11-03 - 17:47 JimSkon  
Pngpng memsel.png manage 3.5 K 2014-11-02 - 14:15 JimSkon  
Pngpng microsequencer.png manage 4.9 K 2014-10-27 - 05:39 JimSkon  
Pngpng microsequencerlab9.png manage 17.9 K 2014-11-02 - 14:08 JimSkon MicroSeqFinal
Topic revision: r22 - 2014-11-12 - JimSkon
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