CS107 Handout 06
Spring 2008 April 4, 2008
Computer Memory: Bits and Bytes
This handout was written by Nick Parlante and Julie Zelenski.
To begin, we are going to take a glimpse into the inner workings of a computer. The
goal is that by seeing the basics of how the computer and compiler cooperate, you will
better understand how language features work.
Basic Architecture
Almost all modern computers today are designed using the Von Neumann architecture
of 1954. In the Von Neumann architecture, the computer is divided into a Central
Processing Unit, CPU, and memory. The CPU contains all the computational power of
the system while the memory stores the program code and data for the program. Von
Neumann's innovation was that memory could be used to store both the program
instructions and the program's data. The instructions that constitute a program are laid
out in consecutive words in memory, ready to be executed in order. The CPU runs in a
"fetch-execute" cycle where it retrieves and executes program instructions from
memory. The CPU executes the current instruction, and then fetches and executes the
next instruction, and so on. The sort of instructions the CPU executes are detailed later in
this handout.
Memory
The smallest unit of memory is the "bit". A bit can be in one of two states— on vs. off, or
alternately, 1 vs. 0. Technically any object that can have two distinct states can
remember one bit of information. This has been done with magnets, gear wheels, and
tinker toys, but almost all computers use little transistor circuits called "flip-flops" to store
bits. The flip-flop circuit has the property that it can be set to be in one of two states, and
will stay in that state and can be read until it is reset.
Most computers don't work with bits individually, but instead group eight bits together
to form a "byte". Each byte maintains one eight-bit pattern. A group of N bits can be
arranged in 2N different patterns. So a byte can hold 28 = 256 different patterns. The
memory system as a whole is organized as a large array of bytes. Every byte has its
own "address" which is like its index in the array. Strictly speaking, a program can
interpret a bit pattern any way it chooses. By far the most common interpretation is to
consider the bit pattern to represent a number written in base 2. In this case, the 256
patterns a byte can hold map to the numbers 0..255.
The CPU can retrieve or set the value of any byte in memory. The CPU identifies each
byte by its address. For this class, we will write memory operations like array
operations, so something like the notation Mem[20]=34 sets the value of memory at
address 20 to the value 34. The byte is sometimes defined as the “smallest addressable
