CPU is the most important part of a computer. It is controlled by
a series of small operations called micro-operations.
You have got a feel in the last two labs with a one-bit CPU, a two-bit CPU and an 8-bit CPU with a memory.
In this lab, we will see how we can use an 8-bit CPU with a memory.
opcode# MNEMONIC DESCRIPTION
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00 XOR source, destination XOR the contents of source,
destination, and store the
result in the destination
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01 AND source, destination AND the contents of source,
destination, and store the
result in the destination
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10 NOT source, destination invert the contents of
source and store the result
in destination
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11 OR source, destination OR the contents of source,
destination, and store the
result in the destination
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Two bits are assigned for the addressing mode and one bit is specified
for selecting register B or C(B=0, C=1) if the addressing mode is register.
1) NOT 2,B ; NOT memory location 2 and store the result in
register B. The machine language code for it is as follows:
10110000
00000010
2) XOR #5,(C) ; XOR 5 and the contents of the memory location
stored in C and store the result in the memory location
stored in C. The machine language code for it is as follows:
00010101
00000101
Fetch Cycle
Before an opcode can be interpreted it must be loaded into the IR.
With our circuit this can be done with the following microcode:
t1: PCout, MARin. ; set up memory address
t2: Set MEMio to 0, MEMenable to 1, IRin.;fetch opcode and store in IR
t3: PC+1out,PCin. ;increment PC
Indirect Cycle
Once an opcode is fetched any operands must be fetched.
The NOT operation, for example, has two operands:
the source and the destination.
The type of fetch will depend on the addressing mode used.
The following microinstructions can be used to fetch the
operand for immediate addressing.
t1: PCout, MARin. ;set up to fetch operands address
t2: Set MEMenable to 1, MEMio to 0, ACCin. ;move operand to ACC
t3: PC+1out, PCin. ;increment PC
t4: ALUout. ;get the result onto the bus
The first operand is loaded into the accumulator(ACC).
The second operand would be loaded into the TEMP register.
An example for absolute addressing mode is:
t1: PCout, MARin. ;set up to fetch operands address
t2: Set MEMenable to 1, MEMio to 0, TMPin. ;move operand address to TMP
t3: PC+1out, PCin. ;increment PC
t4: TMPout, MARin. ;prepare to get operand
t5: Set MEMenable to 1, MEMio to 0, TMPin. ;get operand
The indirect cycle for register mode and indirect mode
have not been shown but the reader should be able to see
how they could be implemented.
Execute Cycle
Where the results are stored is different for each
opcode the CPU can execute. When the destination's
"mode" is indirect or absolute (for example, the
XOR from above), the results are stored in memory.
The following microinstructions are used:
t1: ALUout,
set MEMenable to 1, MEMio to 1, double click on the MEMclk,
to store result in destination.
Note that:
MAR is assumed to contain the destination address
Let's execute NOT 2,B ;
It negates the content in memory location 2 and store the result in register B.
Here is the machine language code for it:
10110000
00000010
Pre-load 00001000 to memory location 2
Select 0x02(i.e. b00000010) from ETXin, EXTout, MARin; then close buffer
Select 0x08(i.e. b00001000) from EXTin, EXTout,
Set MEMio to 1 to store value to the memory,
Set MEMenable to 1 to be able to access the memory,
double click on MEMclk; then close the access to the memory
LOAD MACHINE CODE TO THE MEMORY:
Similarly, Pre-load 10110000 to memory location 0;
Similarly, Pre-load 00000010 to memory location 1;
RUN THE PROGRAM:
1. Make PC point to the beginning of your program
Select 0x00(i.e. b00000000) from ETXin, EXTout, PCin; then close buffer
2. Fetch the instruction
PCout, MARin, MEMenable to 1, MEMio to 0, IRin, MEMenable to 0;
3. Increment PC
PC+1out, PCin; then close PC+1out
4. Fetch the address of the operand (absolute mode)
PCout, MARin, MEMenable to 1, MEMio to 0, TEMin, MEMenable to 0;
5. Fetch the operand
TMPout, MARin, MEMenable to 1, MEMio to 0, ACCin, MEMenable to 0;
6. Get the result
ALUout, Bin; close ALUout
7. Show result on the bus
Bout; showing the result 11110111
With an 8-bit CPU, you can have a RAM attached to it so that you can run some program in it. You may have noticed that many devices may be connected to a common bus by using tri-state buffers.
Copyright: Department of Computer Science, University of Regina.