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Microprocessor
Dr. Rabie A. RamadanAl-Azhar University
Lecture 4
Machine Cycles and Bus Timings
Microprocessor includes 158 different
instruction types.
• Each instruction has two parts:
• Operation code (known as opcode) and operand.
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OUT (10H), A
Opcode to
output data
Operand to specify that the
byte should be sent from the
accumulator to port 10H
Machine Cycles and Bus Timings
Assume that the instruction is stored in 2 Bytes
The Z80 has to perform three operations:
• Read Byte 1 from the first memory location
• Read Byte 2 from the next memory location
• Send Data to port 10H
3
OUT (10H), A
Opcode to
output data
Operand to specify that the
byte should be sent from the
accumulator to port 10H
Machine Cycles and Bus Timings Instruction
• the time required to complete the execution of an instruction. The Z80
instruction cycle consists of one to six machine cycles or one to six
operations.
Machine Cycle
• The time required to complete one operation of accessing memory,
accessing I/O, or acknowledging an external request. This cycle may
consist of three to six T-states.
T-state
• One subdivision of operation performed in one clock period. These
subdivisions are internal states synchronized with the system clock and
each T-state is equal to one clock period.
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Opcode Fetch Machine Cycle (M1)
Address Machine Code Instruction Comment
2002 010001112 (47H) LD B, A Copy A into B
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Opcode Fetch Machine Cycle (M1)
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Opcode Fetch Machine Cycle (M1)
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The Timing of the opcode Fetch Machine
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The Timing of the
opcode Fetch Machine
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The Timing of the
opcode Fetch
Machine
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The Timing of the
opcode Fetch
Machine
Memory Read Machine Cycle
The instruction consists of two bytes;
• Opcode and Data byte.
• The Z80 must first read these bytes from memory
• Requires at least two machine cycles.
• The machine cycle is opcode fetch, and
• the second machine cycle is Memory Read
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Memory Write Cycle
This is a one-byte instruction with two machine cycles:
• Opcode Fetch and Memory Write.
• Z80 fetches the code (77H) and
• it copies the byte 9FH from the accumulator into the memory
location 2350H.
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Memory Interfacing
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Interfacing Memory
An address should be placed on the
address lines.
The low-order address lines are
decoded by the internal decoder of the
memory chip, and the addressed
register is identified.
The high-order address should be
decoded to generate a Chip Select
signal, and the memory chip is
selected by asserting the Chip Select
low.
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CS
Interfacing Memory
To read from the addressed register, the
should be asserted low to enable the
output buffer, and then the data byte
from the register will be placed on the
I/O lines.
To write into the addressed register, the
should be asserted low to enable the
input buffer, and then data bits from the
data lines are stored into the register.
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RD
WR
How does the Z80 Read from or
Write into Memory?
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Basic Concepts in Memory
Interfacing
Interface Function:
• Be able to select the chip
• Identify the register
• Enable the appropriate buffer.
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Read from the Memory
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Write into memory register
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Address Decoding
3 input lines A0-A2 (Memory Select Buffer (MSB))
A3 – A7 are control lines since the address starts at F0H to F7H
3 lines enable only one o/p line
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A7 A6 A5 A4 A3 A2 A1 A0
1 1 1 1 0 1 1 1= F7H
Example 1: Interfacing the 2764
EPROM
Used in industry to develop
microprocessor-based products.
8k (8192 8) memory chip with
8 data lines
Housed in a 28-pin package.
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Chip Configuration
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What is the Memory Addresses Range?
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A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1
0000H 1FFFH
Interfacing CMOS 6116 Static R/W
Memory
This is a 2k static memory chip
organized as 2048 x 8 format.
It has 11 address lines (A10-A0),
8 data lines
3 control signals:
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Chip Configuration
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Memory Interface
What is the addresses range? Assume A12 and A11 are do not care .
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A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1 A0
1 0 0 X X 0 0 0 0 0 0 0 0 0 0 0
1 0 0 X X 1 1 1 1 1 1 1 1 1 1 1
8000 87FF -- Address ranges may differ due to the do not care at A12 and A11
Reading Assignment
Please Read
• Chapter 3
• Chapter 4
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