8085 machine cycle and bus timing diagrams
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8085 machine cycle and bus timing diagrams
• 74 instructions • Each has two parts:operation code and
operand.• Machine cycle:time required by the MP to
complete the operation of accessing memory or i/o device
• Operations like opcode fetch ,M read,M write,i/o read,i/o write are performed in a machine cycle.
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MP EXTERNAL COMMUNICATION FUNCTIONS
• MEMORY READ AND WRITE• I/O READ AND WRITE• REQUEST ACKNOWLEDGE
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• EACH INTRUCTIONS HAS one or more machine cycles
• Each machine cycle is divided into T-states• MP perform an operation in specific time
period (clock cyle) called T-state.• The no of T-states required to perform an
operation is also called machine cycle
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Opcode fetch machine cycle
Each instruction of the processor has one byte opcode. • The opcodes are stored in memory. So, the processor executes the
opcode fetch machine cycle to fetch the opcode from memory. • Hence, every instruction starts with opcode fetch machine cycle. • The time taken by the processor to execute the opcode fetch cycle
is 4T. • In this time, the first, 3 T-states are used for fetching the opcode
from memory and the remaining T-states are used for internal operations by the processor.
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Timing Diagram for Opcode Fetch Machine Cycle
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Memory Read Machine Cycle of 8085:
The memory read machine cycle is executed by the processor to read a data byte from memory.
• The processor takes 3T states to execute this cycle.
• The instructions which have more than one byte word size will use the machine cycle after the opcode fetch machine cycle.
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Memory Write Machine Cycle of 8085:
• The memory write machine cycle is executed by the processor to write a data byte in a memory location.
• The processor takes, 3T states to execute this machine cycle.
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I/O Read Cycle of 8085
• The I/O Read cycle is executed by the processor to read a data byte from I/O port or from the peripheral, which is I/O, mapped in the system.
• The processor takes 3T states to execute this machine cycle.
• The IN instruction uses this machine cycle during the execution.
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I/O Write Cycle of 8085:
• The I/O write machine cycle is executed by the processor to write a data byte in the I/O port or to a peripheral, which is I/O, mapped in the system.
• The processor takes, 3T states to execute this machine cycle.
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