Lecture Overview – September 30, 2015
• Housekeeping
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Questions about second assignment
Questions about first lab
Second lab will be available by week’s end
Third assignment available by Monday
• Lectures and course material
– Pace of the class
– Questions about previous lectures
• Writing code in assembler
– Review of what we’ve done
– Writing a few more complex programs
• Glimpses at computers throughout history
• Computer circuitry from logic gates to silicon
• Moore’s Law
A very simple computer (instruction set)
Operation
Operation code
Stop
0
Load contents of memory location to accumulator
1
Store contents of accumulator in memory location
2
Add contents of memory location to accumulator
3
Print contents of accumulator
4
Get input from keyboard to accumulator
5
A very simple computer (architecture)
CPU
arithmetic, logic, control
GET
accumulator
PRINT
LOAD
STORE
Memory (RAM)
ADD
keyboard
display
A program to print a number
A program to print a number
GET
get a number from keyboard into accumulator
PRINT
print the number that's in the accumulator
STOP
A program to add any two numbers
GET
get first number from keyboard to accumulator
STORE NUM save value in RAM location labeled "NUM"
GET
get second number from keyboard to accumulator
ADD NUM
add value from NUM (1st number) to accumulator
PRINT
print the result (from accumulator)
STOP
NUM --a place to save the first number
• See the code work
Adding 3 numbers
Read
number
Store
at A
Read
number
Store
at B
Read
number
Add A
Add B
Print
result
Stop
A program to add any three numbers
GET
get first number from keyboard into accumulator
STORE NUM1 save value in RAM location labeled "NUM1"
GET
get second number from keyboard into accumulator
ADD NUM1
add value from NUM1 (1st number) to accumulator
STORE NUM2 save value in RAM location labeled "NUM2"
GET
get third number from keyboard into accumulator
ADD NUM2
add value from NUM2 (1st number) to accumulator
PRINT
print the result (from accumulator)
STOP
NUM1 --a place to save the first number
NUM2 --a place to save the second number
questions:
how would you extend this to adding 1000 numbers?
or an unknown number of numbers?
Adding numbers until the number 0 is seen
Read number
Is
It
0?
No
Add to Sum
YES
Print result
Stop
Operations needed for allow reuse of instructions
• Have to be able to go back to a previous instruction
• Have to be able to stop the repetition based on a test
Instructions to allow reuse of instructions
• Have to be able to go back to a previous instruction
– GOTO
take next instruction from a specified RAM location
instead of just using next location
• this lets us repeat a sequence of instructions indefinitely
• Have to be able to stop the repetition based on a test
– IFZERO
if accumulator value is zero, go to specified location
instead of using next location
• these two instructions let us write programs that repeat
instructions until a specified condition becomes true
• the CPU can change the course of a computation according to
the results of previous computations
Add up a lot of numbers and print the sum
Start
Show
Sum
GET
get a number from keyboard
IFZERO Show
if number was zero, go to "Show"
ADD Sum
add Sum so far to new number
STORE Sum
store it back in Sum so far
GOTO Start
go back to "Start" to get the next number
LOAD Sum
load sum into accumulator
PRINT
print result
STOP
0
initial value set to 0 before program runs (by assembler)
Adding a fixed number of numbers
Read number Count
Set Sum to 0
Set I to 1
Is
I <= Count
No
YES
Read a number
and Add to Sum
Add 1 to I
Print Sum
Stop
Add up a number of numbers and print the sum
Start
GET
STORE Count
LOAD Count
SUB
I
IFZERO Show
get a number from keyboard
number of numbers to add
Test to see if we are done
LOAD I
ADD 1
STORE I
Show
Sum
I
GET
get a number from keyboard
ADD Sum
add Sum so far to new number
STORE Sum
store it back in Sum so far
GOTO Start
go back to "Start" to get the next number
LOAD Sum
load sum into accumulator
PRINT
print result
STOP
0
initial value set to 0 before program runs (by assembler)
1
initial value set to 1 before program runs (by assembler)
A simulator for the toy computer
• simulator (a program) reads a program written for the toy computer
• simulates what the toy computer would do
• toy machine's instruction repertoire:
get
print
load Val
store Lab
add Val
sub Val
goto Lab
ifpos Lab
ifzero Lab
stop
M Num
read a number from the keyboard into accumulator
print contents of accumulator
load accumulator with Val (which is unchanged)
store contents of accumulator into location labeled Lab
add Val to accumulator
subtract Val from accumulator
go to instruction labeled Lab
go to instruction labeled Lab if accumulator positive (>= 0)
go to instruction labeled Lab if accumulator is zero
stop execution
before program runs, set this memory location to Num
if Val is a name like Sum, it refers to a memory location with that label;
if Val is a number like 17, that value is used literally
Scaling to real computers
• Our machine had limited RAM
• Word length
– Each of our addresses stored 4 bytes
Was a 32 bit (4 bytes x 8) machine
• Accumulator
– Our machine had 1 accumulator
• Instruction set
– We’re up to 8 instructions
Scaling to real computers
• Our machine had limited RAM
– Current machines have 4,8,16 or more GB of memory
• Word length
– Each of our addresses stored 4 bytes
Was a 32 bit (4 bytes x 8) machine
– Current machines typically store words of 8 bytes (64 bits)
64 bit addresses are needed. Why??
• Accumulator
– Our machine had 1 accumulator
– Current machines have 16 (or more) accumulators (called registers)
• Instruction set
– We’re up to about 8 instructions
– Current machines have 256 instructions, many with sub-instructions
Key architectural observation
• We store instructions and data together
– So called Von Neumann architecture
• There is a risk
– Imagine reading a data address as an instruction
• Looping is a very powerful construct
– Beware the infinite loop
• There are many benefits
– Programs can modify themselves productively
Pause for interesting website
• The way back machine (https://archive.org/web/)
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Princeton.edu
Nytimes.com
Dailyprincetonian.com
Goprincetontigers.com
….
A brief history of computing devices
• Analog devices
– Largely aimed at solving individual problems
• Devices to win World War 2 (the first digital devices)
– British machines at Bletchley Park
• Post War devices
– Largely done as government-industry-university collaborations in the
US
• The rise of personal computers
Babbage – Analytical Engine (1837)
Could store 1000 40
digit decimal numbers
(16.7kB)
Was programmable
Was never built
Ada Lovelace (nee Byron)
the first programmer
Differential analyzer (Bush and Hazen 1931)
Colossus 1943
• Built to break codes
• Was the first programmable (though by plugs and switches) electronic
digital computer
Bombe
• Designed by Alan Turing
starting in 1939
• Electromechanical switching
• Built to emulate the German
enigma machine (code breaking)
IBM Automatic Sequence Controlled
Calculator (ASCC) (Harvard) 1944-1959
Input/Output devices
ENIAC, 1946
Size 3’x8’x100’; weight 30 tons; clock speed 5kHz;
could store ~ 30 10 digit numbers
Penn
John von Neumann and the Johnniac (1953)
Von Neumann
architecture was
first used on the
IAS machine (1951).
Johnniac had 1024 words (40 bits each) of memory. Was used 1953—66.
Two paths to personal computers
• Xerox  Apple
• Altair  IBM PC
Xerox PARC Alto 1973
Apple II 1977
Apple Lisa 1982
Macintosh 1984
First personal computer for sale
Altair 8800 (1975)
Sold for $439 as a kit, $621 assembled
4K RAM; 2 MHz clock rate
Altair 8800 (1975)
Programming the Altair was an extremely tedious process. The
user toggled the switches to positions corresponding to an 8080
microprocessor instruction or opcode in binary, then used an
'enter' switch to load the code into the machine's memory, and
then repeated this step until all the opcodes of a presumably
complete and correct program were in place. When the machine
first shipped the switches and lights were the only interface,
and all one could do with the machine was make programs to
make the lights blink. (Wikipedia)
First major piece of software for Altair
• BASIC programming language
IBM PC 1981
http://www.dailydot.com/technology/int
ernet-speed-intelligence/