Page 26 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 2: Data Processing

depending on the details of the particular computer. The program counter is automatically Notes
incremented for each machine cycle so that instructions are normally retrieved sequentially
from memory.
The control unit places these instructions into its instruction register and then increments the
program counter so that it contains the address of the next instruction stored in memory. It then
executes the instruction by activating the appropriate circuitry to perform the requested task.
As soon as the instruction has been executed, it restarts the machine cycle, beginning with the
fetch step.
During one machine cycle the processor executes at least two steps, fetch (data) and execute
(command). The more complex a command is (more data to fetch), the more cycles it will take
to execute. Reading data from the zero page typically needs one cycle less as reading from an
absolute address. Depending on the command, one or two more cycles will eventually be needed
to modify the values and write them to the given address.
Control Unit is the part of the computer that controls the Machine Cycle.

It takes numerous cycles to do even a simple addition of two numbers.
Fetch - get an instruction from Main Memory
Decode - translate it into computer commands

Execute - actually process the command
Store - write the result to Main Memory
It has to be considered how long a specific command takes to execute when coding, so that the
whole timing inside the program altogether and specially the screen output works. This is referred
to as “cycle counting”. Beginners in assembler programming often throw in NOP commands
(takes two cycles) to achieve a stable timing.

2.3 Memory

There are two kinds of computer memory: primary and secondary. Primary memory is an integral
part of the computer system and is accessible directly by the processing unit. RAM is an example of
primary memory. As soon as the computer is switched off the contents of the primary memory is
lost. The primary memory is much faster in speed than the secondary memory. Secondary memory
such as floppy disks, magnetic disk, etc., is located external to the computer. Primary memory
is more expensive than secondary memory. Because of this, the size of primary memory is less
than that of secondary memory. Computer memory is used to store two things: (i) instructions
to execute a program and (ii) data.
When the computer is doing any job, the data that have to be processed are stored in the primary
memory. This data may come from an input device like keyboard or from a secondary storage
device like a floppy disk. As program or the set of instructions is kept in primary memory, the
computer is able to follow instantly the set of instructions.
2.3.1 Primary Memory

The primary memory in the computer is in the form of IC’s (Integrated Circuits). These
circuits are called Random Access Memory (RAM). Each of RAM’s locations stores one byte
of information. (One byte is equal to 8 bits). A bit is an acronym for binary digit, which
stands for one binary piece of information. This primary or internal storage section is made
up of several small storage locations (ICs) called cells. Each of these cells can store a fixed
number of bits called word length. Each cell has a unique number assigned to it called the

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