Structure and functions of a microcomputer
Basic Units
Figure 1 shows the structure of a simple microprocessor-based microcomputer. The microprocessor consists of three basic units, which perform the following functions: synchronization of processing events and instruction decoding (control unit); temporary storage of addresses and data (registers); arithmetic, logic and shift operations (arithmetic unit).
Program instructions and data are held in memory (RAM, ROM, PROM, EPROM, etc.) until fetched by the control unit signals. Communication to the outside world to various types of peripherals is via one or more input/output ports.
The processing events, controlled by the control unit, are triggered by a quartz crystal clock, which generates pulses at regular intervals depending on its frequency. For example, a clock with a frequency of 1 MHz (Megahertz) has a period of 1ms (microsecond). The clock logic (for driving the clock) may be on the microprocessor chip itself or external to it.
The units are linked by electrical lines, which carry electrical pulses representing memory and input/output port addresses (address bus) and data (data bus). A typical 8-bit microprocessor has a 16-line address bus for carrying 2-byte address, and an 8-bit data bus for carrying 1-byte data words. Synchronization signals for controlling the processing events are carried by control lines (control bus). The registers, arithmetic and logic unit circuits and memory devices are isolated from the buses by three-state buffers (not shown). Timing and control circuitry is used to ensure that only the device that is transmitting or receiving data is connected to the appropriate bus at any given time.
Microcomputing devices require a small power supply, typically 5v or three levels (+5v, +12v, -5v), depending on the particular devices being used.
At the beginning of a program run, the program counter is set to the address of the first instruction to be executed, subsequently it will be set to the next instruction to be executed.
Control
ALU Registers unit
timing/ control/ decoding circuits
adder/ comparator circuits
accumulators
stack
pointer
program
counter status
flags
Index registers
Microprocessor
address
bus
RAM
ROM
input/ output parts
data bus
control bus
Figure 1. Structure of a simple microcomputer.
Status flags are single-bit registers, which are set or reset automatically according to the results of arithmetic operations; they can be tested by program instructions. Typical status flags are N, Z and V, which record respectively whether the result of an arithmetic operation was negative, zero or was too large to be stored correctly (overflowed). Another flag commonly used is the carry flag (C), which records whether a carry occurred on the left of an addition or subtraction. In each case, the status flag is set to 1 if the specified condition has occurred; otherwise it is set to 0.
Accumulators are registers, which are used to store data that has been fetched from memory. Program instructions are available, which operate on data held in accumulator. One or more accumulators are available depending on the particular microprocessor being used.
Index registers are used to hold values, which can be used to modify actual memory addresses to produce effective addresses. In this way, the results of previous processing in the program run can be used to alter the sequence in which instructions are obeyed.
Part of the RAM memory of typical microprocessor-based microcomputers is reserved for stack operations. The stack is used, by means of program instructions, to store data temporarily for subsequent retrieval. The data is pushed on to the stack on byte at a time. On retrieval, the topmost byte is pulled off first, that is, the stack operates in LIFO mode (Last-In, First-Out).
Chip or bit slice-based microcomputers function in a similar way to microprocessor-based microcomputers. The essential difference is that the computer units may be made up from a number of different chips. For example, the functions of the microprocessor chip can be implemented on several chips slices to overcome the limitation of a particular microprocessor and give improved performance. This philosophy can be extended to the other computer units.
TASKS
1. Give equivalents of the following words and transcribe them
synchronization (n.)
programmable (adj.)
access (n.)
erasable (adj.)
various (adj.)
peripheral (n.)
trigger (v.)
quartz (adj.)
frequency (n.)
megahertz (n.)
isolate (v.)
buffer (n.)
transmit (v.)
appropriate (adj.) (v.)
execute (v.)
subsequent (adj.)
subsequently (adv.)
reset (v.)
record (v.) (n.)
occur (v.)
accumulator (n.)
available (adj.)
value (n.)
alter (v.)
sequence (n.)
retrieval (n.)
push on (v.)
pull off (v.)
implement (v.)
extend (v.)
2. Give equivalents of the following word combinations
synchronization of processing events
instruction decoding
control unit
temporary storage
arithmetic operations
logic operations
shift operations
arithmetic unit
program instructions
RAM (Random Access Memory)
ROM (Read-Only Memory)
PROM (Programmable Read-Only Memory)
EPROM (Erasable Programmable Read-Only Memory)
input/output ports
quartz crystal clock
clock logic
microprocessor chip
address bus
data bus
control bus
three-state buffer
power supply
program counter
status flag
single-bit registers
carry flag
index register
actual memory addresses
effective addresses
stack operations
LIFO mode (Last-In, First-Out)
chip or bit slice-based microcomputers
3. Under the letter c give definitions based on the text.
1) A control unit a) refers to those parts of the computer that dictate the functions
to be performed by all other parts.
b) is a device for causing the machine to perform the desired
operations in the correct order.
c)
2) An arithmetic unit a) is that part of the computer which can carry out
arithmetic operations.
b) is a device for performing calculations.
c)
4. Complete the definitions
Registers (memory cells) are groups of memory elements used for _____ .
Input/output ports are devices used for________ .
A quartz crystal clock is a device used to trigger_____ .
A clock logic is a device which _______ .
An address bus represents _________ .
A data bus represents _________ .
A control bus refers to __________ which synchronize _________ .
Three-state buffers are devices used to isolate ___________ .
Timing and control circuits are used to ensure ____________ .
10. Status flags are ___________ .
11. A carry flag records ______ .
12. Accumulators are registers used ________ .
13. Index registers are registers for holding ___________ .
14. A stack is a device used ________ .
5. Read and translate the text.
TEXT 2
STRUCTURE AND FUNCTIONS OF A MICROCOMPUTER