The CPU in more detail

1
The CPU in more detail

• Programs and data are stored in memory
• Memory usually refers to RAM, but we use the more
  general term memory to also include the caches
• The CPU executes program instructions and
  processes data
• The CPU performs the following
• Fetches instructions and data from memory
• Decodes and executes instructions
• Stores results back into memory (to be displayed
  on the screen or stored onto disk or)
• Goes to the next instruction (if there is one)

2
CPU High-Level Layout
CPU
Instructions and data go from Mem to CPU
Memory(or memory hierarchy)
Registers
Data Bus
ControlUnit
Processed data (results) go from CPU to Mem
ALU
Memory contains data and instructions
CPU contains a control unit, the ALU, and
registers
3
CPU a closer look
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
03
PC Program Counter
R1 General Purpose Register 1
04
Data Bus
00
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR M6 R1
200
07
AC Accumulator
08

• Instruction cycle execution of a single program
  instruction by the CPU
• Fetch control unit loads instruction from memory
  location in PC into IR
• Decode control unit interprets instruction
• Execute control unit executes instruction (tells
  ALU what to do)
• PC is incremented to next program instruction and
  cycle repeats

4
Example Instruction Set

• Instruction set basic set of operations the CPU
  can perform

Op Code and Usage Operation
INP val mem Puts a number value (val) in a particular memory address (mem)
MAM mem Copy value in accumulator to memory address (mem)
MMR mem reg Copy value at memory address (mem) to register (reg)
MRA reg Copy value in register (reg) to accumulator
MAR reg Copy value in accumulator to register (reg)
ADD reg1 reg2 Add values in reg1 and reg2, put result in accumulator
SUB reg1 reg2 Subtract value in reg2 from reg1, put result in accumulator
MUL reg1 reg2 Multiply values in reg1 and reg2, put result in accumulator
DIV reg1 reg2 Divide value in reg1 by reg2, put result in accumulator
JMP mem Unconditional jump to the instruction in memory address (mem), PCmem-1
JPZ mem Jump to the instruction in memory address (mem) if accumulator value 0
JPN mem Jump to instruction in memory address (mem) if accumulator value is NOT 0
JPL mem Jump to instruction in memory address (mem) if accumulator value is lt 0
JPG mem Jump to instruction in memory address (mem) if accumulator value is gt 0
HLT Halts program execution, indicates end of a program
5
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
00
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
07
AC Accumulator
08

• Lets start with a simple program that will add
  two values in memory and store the result in
  another location in memory.

6
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
00
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
07
AC Accumulator
08

• Program begins by the operating system
• Loading the instructions and data into memory
• Setting the program counter to the starting
  instruction (ex PC0)
• The CPU is always performing the
  Fetch-Decode-Execute cycle, so the program then
  begins immediately

7
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
00
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR 06 R1
200
07
AC Accumulator
08

• First pass of Fetch-Decode-Execute instruction
  cycle
• Fetch instruction at memory location in PC (00)
  is loaded into IR

8
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
00
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR 06 R1
200
07
AC Accumulator
Move memory value to register
08

• First pass of Fetch-Decode-Execute instruction
  cycle
• Fetch instruction at memory location in PC (00)
  is loaded into IR
• Decode control unit decodes the message

9
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
00
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR 06 R1
200
07
AC Accumulator
Move memory value to register
08

• First pass of Fetch-Decode-Execute instruction
  cycle
• Fetch instruction at memory location in PC (00)
  is loaded into IR
• Decode control unit decodes the message
• Execute control unit moves value in memory to a
  register

10
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
01
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR 06 R1
200
07
AC Accumulator
08

• First pass of Fetch-Decode-Execute instruction
  cycle
• Fetch instruction at memory location in PC (00)
  is loaded into IR
• Decode control unit decodes the message
• Execute control unit moves value in memory to a
  register
• Increment program counter (PC) to next
  instruction (PC01)

11
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
01
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR 07 R2
200
07
AC Accumulator
08

• 2nd instruction cycle
• Fetch instruction at memory location in PC (01)
  is loaded into IR

12
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
01
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
MMR 07 R2
200
07
AC Accumulator
Move memory value to register
08

• 2nd instruction cycle
• Fetch instruction at memory location in PC (01)
  is loaded into IR
• Decode control unit decodes the message

13
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
01
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
MMR 07 R2
200
07
AC Accumulator
Move memory value to register
08

• 2nd instruction cycle
• Fetch instruction at memory location in PC (01)
  is loaded into IR
• Decode control unit decodes the message
• Execute control unit moves value in memory to a
  register

14
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
02
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
MMR 07 R2
200
07
AC Accumulator
08

• 2nd instruction cycle
• Fetch instruction at memory location in PC (01)
  is loaded into IR
• Decode control unit decodes the message
• Execute control unit moves value in memory to a
  register
• Increment program counter (PC) to next
  instruction (PC02)

15
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
02
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
ADD R1 R2
200
07
AC Accumulator
08

• 3rd instruction cycle
• Fetch

16
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
02
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
ADD R1 R2
200
07
AC Accumulator
Add values in registers, store in accumulator
08

• 3rd instruction cycle
• Fetch
• Decode

17
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
02
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
ADD R1 R2
200
07
AC Accumulator
Add values in registers, store in accumulator
08
300

• 3rd instruction cycle
• Fetch
• Decode
• Execute

18
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
03
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
ADD R1 R2
200
07
AC Accumulator
08
300

• 3rd instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

19
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
03
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
MAM 08
200
07
AC Accumulator
08
300

• 4th instruction cycle
• Fetch

20
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
03
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
MAM 08
200
07
AC Accumulator
Move value in accumulator to memory
08
300

• 4th instruction cycle
• Fetch
• Decode

21
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
03
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
MAM 08
200
07
AC Accumulator
Move value in accumulator to memory
300
08
300

• 4th instruction cycle
• Fetch
• Decode
• Execute

22
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
04
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
MAM 08
200
07
AC Accumulator
300
08
300

• 4th instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

23
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
04
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
HLT
200
07
AC Accumulator
300
08
300

• 5th instruction cycle
• Fetch

24
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
04
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
HLT
200
07
AC Accumulator
End of the program!
300
08
300

• 5th instruction cycle
• Fetch
• Decode

25
Example Program Execution
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
MMR 06 R1
00
MMR 07 R2
01
ADD R1 R2
02
MAM 08
03
PC Program Counter
R1 General Purpose Register 1
HLT
04
Data Bus
04
100
05
R2 General Purpose Register 2
IR Instruction Register
100
06
200
HLT
200
07
AC Accumulator
End of the program!
300
08
300

• 5th instruction cycle
• Fetch
• Decode
• Execute ask OS for new PC value (next program to
  run)

26
More Complex Example Loop

• Lets compute power of 2!
• Lets sketch the algorithm needed
• Use two variables X and Y(X will store our
  powers and Y will store the multiplier 2)
• Initialize X1, Y2
• Repeat the following operations X XY
• Will this work? Yes!
• Begin X1, Y2
• Loop1 XXY X2, Y2
• Loop2 XXY X4, Y2
• Loop2 XXY X8, Y2
• how do we make it stop? Worry about that later

27
More Complex Example Loop

• Lets write the assembly program
• 00 Storage for starting X (1)
• 01 Storage for multiplier Y (2)
• 02 Storage for Result (powers of 2)
• 03 INP 1 00 // PUT VALUE 1 IN MEM00
• 04 INP 2 01 // PUT VALUE 2 IN MEM01
• 05 MMR 00 R1 // COPY MEM00 (1) TO REG1
• 06 MMR 01 R2 // COPY MEM01 (2) TO REG2
• 07 MUL R1 R2 // ACCUMULATOR REG1REG2
• 08 MAR R1 // COPY ACCUMULATOR TO REG1
• 09 MAM 02 // COPY ACCUMULATOR TO MEM02
• 10 JMP 07 // LOOP JUMP TO MEM07

MEM
DATA
PROGRAM
28
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
00
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
03
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
MUL R1 R2
07
AC Accumulator
MAR R1
08
MAM 02
09
JMP 07
10

• OS loads program into memory
• OS sets program counter, PC03
• Program execution begins!

29
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
04
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
INP 1 00
MUL R1 R2
07
AC Accumulator
Put value 1 into MEM00
MAR R1
08
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

30
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
05
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
INP 2 01
MUL R1 R2
07
AC Accumulator
Put value 2 into MEM01
MAR R1
08
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

31
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
06
1
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
MMR 00 R1
MUL R1 R2
07
AC Accumulator
Copy MEM00 to R1
MAR R1
08
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

32
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
07
1
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MMR 01 R2
MUL R1 R2
07
AC Accumulator
Copy MEM01 to R2
MAR R1
08
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

33
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
08
1
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MUL R1 R2
MUL R1 R2
07
AC Accumulator
AC R1R2
MAR R1
08
2
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

34
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
09
2
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MAR R1
MUL R1 R2
07
AC Accumulator
Copy AC to R1
MAR R1
08
2
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

35
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
2
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
10
2
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MAM 02
MUL R1 R2
07
AC Accumulator
Copy AC to MEM02
MAR R1
08
2
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

36
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
2
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
07
2
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
JMP 07
MUL R1 R2
07
AC Accumulator
Jump to MEM07PC07
MAR R1
08
2
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

37
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
2
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
08
2
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MUL R1 R2
MUL R1 R2
07
AC Accumulator
AC R1R2
MAR R1
08
4
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

38
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
2
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
09
4
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MAR R1
MUL R1 R2
07
AC Accumulator
Copy AC to R1
MAR R1
08
4
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

39
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
4
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
10
4
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MAM 02
MUL R1 R2
07
AC Accumulator
Copy AC to MEM02
MAR R1
08
4
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

40
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
4
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
07
4
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
JMP 07
MUL R1 R2
07
AC Accumulator
Jump to MEM07PC07
MAR R1
08
4
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

41
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
2
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
08
4
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MUL R1 R2
MUL R1 R2
07
AC Accumulator
AC R1R2
MAR R1
08
8
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

42
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
2
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
09
8
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MAR R1
MUL R1 R2
07
AC Accumulator
Copy AC to R1
MAR R1
08
8
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

43
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
8
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
10
8
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
MAM 02
MUL R1 R2
07
AC Accumulator
Copy AC to MEM02
MAR R1
08
8
MAM 02
09
JMP 07
10

• Fetch-Decode-Execute instruction cycle
• Fetch
• Decode
• Execute
• Increment PC

44
More Complex Example Loop
CPU
Memory
ALU Performs arithmetic and logic ops as
instructed by CU
Control Unit Fetches Decodes Executes instructions
1
00
2
01
8
02
INP 1 00
03
PC Program Counter
R1 General Purpose Register 1
INP 2 01
04
Data Bus
07
8
MMR 00 R1
05
R2 General Purpose Register 2
IR Instruction Register
MMR 01 R2
06
2
JMP 07
MUL R1 R2
07
AC Accumulator
Jump to MEM07PC07
MAR R1
08
8
MAM 02
09
JMP 07
10

• Infinite loop!
• Program writes powers of 2 to MEM02 forever!
• Lets make it terminate when we reach 8

45
More Complex Example Loop

• 00 Storage for starting X and termination value
  (8)
• 01 Storage for multiplier Y (2)
• 02 Storage for Result (powers of 2)
• 03 INP 1 00 // PUT VALUE 1 IN MEM00
• 04 INP 2 01 // PUT VALUE 2 IN MEM01
• 05 MMR 00 R1 // COPY MEM00 (1) TO REG1
• 06 MMR 01 R2 // COPY MEM01 (2) TO REG2
• 07 INP 8 00 // RE-USE MEM00, PUT VAL 8
• 08 MUL R1 R2 // ACCUMULATOR REG1REG2
• 09 MAR R1 // COPY ACCUMULATOR TO REG1
• 10 MAM 02 // COPY ACCUMULATOR TO MEM02
• 11 MMR 00 R2 // COPY MEM00 (8) TO REG2
• 12 SUB R2 R1 // ACCUMULATOR REG2 REG1
• 13 JPZ 16 // IF AC0 (REGREG2) GOTO MEM16
• 14 MMR 01 R2 // COPY MEM01 (2) BACK INTO
  REG2
• 15 JMP 08 // LOOP JUMP TO MEM08
• 16 HLT // END OF THE PROGRAM

MEM
DATA
PROGRAM