Introduction to

Introduction To Computer Science
38
Memory
The computer has a memory that can hold both data and also the program processing that 
data. In modern computers this memory is known as RAM(Random Access Memory). The 
following are items that characterize the Memory (RAM) in the Von Neumann architecture:
– Consists of many memory cells (storage units) of a fixed size. 
– Each cell has an address associated with it: 0, 1, …
– All accesses to memory are to a specified address. 
– A cell is the minimum unit of access (fetch/store a complete cell).
– The time it takes to fetch/store a cell is the same for all cells.
– When the computer is running , both program and data also known as 
variables are stored in the memory.
– Typical memory size in a personal computer (PC) is 4GB- 8GB.
– Typical memory access time or speed in a personal computer is in the 
range of nanoseconds.
– It is volatile in that it can only store when power is on
– Typical operations in memory are:
√ Fetch(address)- fetching a copy of the content of memory cell with the specified 
address.
√ Store(address, value)- storing the specified value into the memory cell specified 
by address.
– The memory is interfaced via:
• Memory Address Register,
• Memory Data Register,
• Fetch/Store signal.
• Thus, the two memory operation fetch and store amounts to:
– Fetch(address)
– Load address into MAR.
– Decode the address into MAR.
– Copy the content of Memory cell with specified address into MDR.
• Store(address, value)
• Load the address into MAR.
• Load the value into MDR.
• Decode the address in MAR.
• Copy the content of MDR into memory cell with the specified address.

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Computer System
39
Input / Output
The input/output component of the Von Neumann architecture handles devices that allow a 
computer system to communicate with other devices such as a screen(or monitor), keyboard, 
printer, etc. It also allows a computer system to store information via storage devices such as 
internal and external hard-drives, CD-ROMs, Memory stick(flash disks), etc. 
There are two types of access methods where storage devices (mass-storage devices) can be 
accessed by a computer system:
a. Direct Access Methods are applicable for hard-drives, CD-ROMs, memory stick 
or flash disks and the like. Direct access means random access is possible.
b. Sequential Access Methods are applicable for tape drives where data can only 
be accessed serially or sequentially. Sequential access means random access is not 
possible
Characteristics of Input /Output (I/O) devices:
a. Compared to Memory or RAM, speed of I/O devices is slow, it is in the range of 
milliseconds
b. In order to mitigate the slowness problem, I/O devices make use of controllers. 
• A controller, which has a special purpose processor has a small memory 
buffer, and a control logic to control I/O device (e.g. to move a disk 
arm).
• This solution enables data transfer between RAM and the memory buffer, 
and accessing data from the memory buffer is faster. 
The ALU
The Arithmetic/ Logic Unit(ALU) performs mathematical operations((+, -, x, /, …) and logic 
operations (=, <, >, and, or, not, ...). The ALU is a sub-component of the CPU(Central 
Processing Unit). The ALU is composed of:
• Circuits responsible for performing arithmetic/logic operations,
• Registers responsible for storing intermediate computational results, 
and
• Bus that connects the two.

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