A program executed by the processor consists of a series of instructions stored in memory. To process
each of these instructions, is by 2 steps:
• Processor reads instructions from memory at a time it fetches
• Execution of execute instructions
To execute a program, there will be a repetition of the instruction fetch and instruction execution
processes.
Processing that requires a single instruction is referred to as an instruction cycle.
An instruction has a format (depending on the type of machine), which consists of 2 main parts, namely
the opcode and address. The opcode is a code for the operation to be carried out, while the address is
the memory address where the data is stored. the instruction format of the Hypothetical machine
There are 3 key characteristics of memory, namely capacity, access time and cost. For these three
characteristics are interrelated, Figure 1.6 presents the relationship in a memory hierarchy, that is, the
lower it is:
a. Cost/bit reduction
b. Capacity building
c. Improved access time
d. Reducing the frequency of access to memory by the processor.
Most of the OS code is dedicated to handling I/O. a general purpose register consists of a CPU and many
device controllers connected by a bus. Each device controller, handles 1 or many types of devices,
depending on the controller. For example, 7 or more devices, can be plugged into the small computer
system interface (SCSI).
A device controller has some local buffer storage and a series of special-purpose registers. The device
controller is responsible for transferring data between the peripheral devices being controlled and the
local buffer storage. The OS usually has a device driver for each device controller. These device drivers
understand the device controller and provide parts of the operating system with a unified interface to
the device.
A modern general-purpose computer system consists of one or many processors and a number of device
controllers connected by a common bus that provides access to shared memory (Figure 1.6). each
device controller will service one type of device (ex, disk drives, audio devices, or video displays).
The CPU and device controllers can run in parallel, and compete for memory cycles. To guarantee the
order of access to shared memory, a memory controller synchronizes access to memory.
