The von Neumann Architecture of Computer Systems

paper by H. Norton Riley

David Morgan

von Neumann archtecture Starting point

computer must store – data – instructions for manipulating it – the results (themselves data)
dichotomy/polarity data  instructions

“To von Neumann, the key to building a general purpose device was in its ability to store not only its data and the intermediate results of computation, but also to store the instructions, or orders, that brought about the computation.” – Riley paper

Form of instructions?

the wiring
special purpose machines – a toaster
heats the element red-hot
“heat the element red-hot” is the only instruction
fixed in the wiring – a (hypothetical) special-purpose computer
adds two numbers
“add the numbers” is the only instruction
fixed in the wiring A special -purpose computer the purpose is adding

3 + 7 = 10

Special -purpose vs general -purpose

dedicated vs versatile
fixed vs malleable
a (hypothetical) general-purpose computer – adds or subtracts or multiplies or divides – “add” “subtract” “multiply” “divide” are the 4 instructions – in the wiring, but the wiring is variable

“In a special purpose machine the computational procedure could be part of the hardware. In a general purpose one the instructions must be as changeable as the numbers they acted upon. Therefore, why not encode the instructions into numeric form and store instructions and data in the same memory?” – Riley paper An instruction encoded into numeric form for a general -purpose computer

numeric form distinguishing

What is the number? Where is the wiring? What is the wiring?

Numeric instruction instructs

what is the number? – the number is 45
where is the wiring? – the wiring is in the instruction register
what is the wiring? – the wiring is the presence of 45 in the register – because the “bits” are electric switches (not red pennies) – because the “numbers” are electron flow configurations (rewirings) – interpret “instructions” as switch configs or as numbers (your choice, according as you’re a hardware engineer or programmer) Hypothetical instruction set

defined:

00000001, in the IR  add 00000010, in the IR  subtract 00000011, in the IR  multiply 00000100, in the IR  divide

00000101, in the IR  what happens?

Critiques of von Neumann

instructions and data distinguished only implicitly through usage
there is a single 1-dimensional memory
meaning of data not stored with it
instruction and data fetches bottleneck on the bus Instructions/data distinguished implicitly

any trouble if instructions get used as if data?
any trouble if data get used as if instructions?
cf. tagged architecture

A single 1-dimensional memory

in disaccord with programming languages
programs have a data space, code space, stack space
mapping separate spaces into a unitary one (addressing) becomes an unavoidable task Meaning/type of data not stored with it

data’s type? (integer, floating point, string?)
implicit high level languages – 33 is an integer, not floating point not string – 3.14 is floating point, not integer not string – hello is string, not integer not floating point
one operation at high level handles all (at low level it is not one operation) – 33+44 – 3.14+2.72 – hello+ there

von Neumann bottleneck

instructions (opcodes) and data (operands) must shuttle memory cpu and back
they have only 1 bus to get there and must share
fetching each precludes the other at any moment