Building a computer
Layers of abstrac<on
abstrac<on building blocks examples
computer components Macbook Pro
components mul<plexer,
demul<plexer, adder, flip-‐flop
registers, ALU, counter
sequen<al circuits logic gates, connectors,
clock flip-‐flop
combina<onal circuit logic gates, connectors mul<plexer,
demul<plexer, adder logic gates switches, connectors AND, OR, NOT
clock raw materials crystal oscillator
connector raw materials wire
switch raw materials transistor, relay
Connector
• Goal: conduct electricity
+ =
wire
big hole miners
Clock
• Goal: periodic signal for synchroniza<on
+
quartz crystal electricity clock signal
=
Switch
• Goal: allow electricity to be turned on & off
+ +
spring magnet wire
=
switch
Logic gates
• Goal: compute simple Boolean func<ons
wire
+ =
switch NOT gate
+ + =
wire switch switch OR gate
Examples: NOT, OR, AND, NOR, NAND, XOR
Combina<onal circuit
• Goal: compute any Boolean func<ons
x y F(x,y)
0 0 0
0 1 1
1 0 1
1 1 0
truth table
+
logic gates
=
combina<onal circuit wire
+
Examples: odd, majority, mul<plexer, demul<plexer,
adder
Sequen<al circuit
• Goal: store persistent state
logic gates wire
+ +
clock
=
sequen<al circuit
Examples: SR latch, D flip-‐flop, JK flip-‐flop, T flip-‐flop
Components
• Goal: provide high-‐level func<onality
combina<onal circuits sequen<al circuits
+ =
component
Examples: register, register bank, mul<port register
bank, synchronous counter, ALU
Computer
• Goal: store and process data according to a program
+
components input
+
output
=
computer
Computer
• Three major components
– Central Processing Unit (CPU)
• Performs opera<ons
– Memory
• Where data / program is stored
– Input / Output (I/O) system
• Get input, show results
Input/output configura<ons
I/O CPU Memory CPU coordinates
transfer between I/O and memory
CPU Memory I/O Direct Memory
Access (DMA)
Bus: wires carrying informa<on between components
8-‐bit bus in Logisim
Bus width
4-‐bit Intel 4004 8-‐bit Intel 8052 16-‐bit Intel 80286 32-‐bit Intel 80386
32-‐bit Intel 80486 32-‐bit Intel 64-‐bit AMD Opteron 128-‐bit ???
Connec<ng CPU & Memory
– Address bus
• Where CPU puts address of memory it wants
– Data bus
• Where memory puts data from requested address
– Control and status bus
• Determines direc<on and coordinates <ming
CPU Memory
address bus
data bus
control and status bus
Performs opera<ons Stores operands and
results
CPU
• Central Processing Unit (CPU)
– The brains of the computer – Main components:
• Arithme<c Logic Unit (ALU)
– Perform math and logic opera<ons
• Control Unit (CU)
– Fetching and decoding instruc<ons from memory
– Execu<ng instruc<ons by direc<ng sequence of transfers and
opera<ons
Central Processing Unit (CPU) Control Unit (CU)
General purpose
registers ArithmeDc and Logic
Unit (ALU)
Program Counter (PC) InstrucDon Register (IR)
Memory address register Memory data register
InstrucDon decoder Control and status bus
Data bus
Address bus
Computer architecture
Our "simple" ALU
First input
Second input
Output of calcula<on
Which type of thing to output, 00 = ADD, 01 = SUB, 10 = AND, 11 = XOR Business end of the
ALU, compute all opera<ons in parallel
2 bits on this line
4 bits on these lines
ALU data paths
ALU
Accumulator (A)
Result replaces old value in accumulator
A B
1
stinput, current
contents of accumulator 2
ndinput, read
from data bus
8052 adding numbers
• Goal:
– Add value in register 0 and 1 – Put result in register 2
• One-‐liner in C:
r2 = r0 + r1;
Example 8052 opcode
• Opcode ADD
– Adds values
InstrucDons OpCode Bytes Cycles Flags
ADD A,#data 0x24 2 1 C, AC, OV
ADD A,direct 0x25 2 1 C, AC, OV
ADD A,@R0 0x26 1 1 C, AC, OV
ADD A,@R1 0x27 1 1 C, AC, OV
ADD A,R0 0x28 1 1 C, AC, OV
ADD A,R1 0x29 1 1 C, AC, OV
ADD A,R2 0x2A 1 1 C, AC, OV
ADD A,R3 0x2B 1 1 C, AC, OV
ADD A,R4 0x2C 1 1 C, AC, OV
ADD A,R5 0x2D 1 1 C, AC, OV
Example 8052 opcode
• Opcode MOV
– Move memory
InstrucDons OpCode Bytes Cycles Flags
MOV A,R0 0xE8 1 1 None
MOV A,R1 0xE9 1 1 None
MOV A,R2 0xEA 1 1 None
MOV A,R3 0xEB 1 1 None
MOV A,R4 0xEC 1 1 None
MOV A,R5 0xED 1 1 None
MOV A,R6 0xEE 1 1 None
MOV A,R7 0xEF 1 1 None
MOV R0,A 0xF8 1 1 None
MOV R1,A 0xF9 1 1 None
MOV R2,A 0xFA 1 1 None
MOV R3,A 0xFB 1 1 None
…
8052 adding numbers
• Goal:
– Add value in register 0 and 1 – Put result in register 2
• Takes sequence of 3 instruc<ons in 8052 assembly:
– Reflec<on of the underlying hardware reali<es
MOV A, R0
ADD A, R1
MOV R2, A
Control Unit
• Control Unit (CU)
– Fetches instruc<ons from memory – Decodes instruc<ons
– Executes instruc<ons
• Sequence of data transfers & opera<ons
Control Unit (CU)
Program Counter (PC) InstrucDon Register (IR)
Memory address register Memory data register
InstrucDon decoder Control and status bus
Data bus
Address bus
