![]() ![]() For example DCX H, PCHL, SPHL, INX H, etc. One byte instructions those operate on sixteen bit data (16 bit operand) are executed in T 5 and T 6. During T 5 and T 6, 8085 performs stack write, internal 16 bit and conditional return operations depending upon the type of instruction. Step 5 : (State T 5 and T 6) State T 5 and T 6, when entered, are used for internal microprocessor operations required by the instruction. Note : For one byte instructions which operate on eight bit data, data is always available in the internal memory of 8085 i.e. One byte instructions those operate on eight bit data (8 bit operand) are executed in T 4.įor example : MOV A, B, ANA D, ADD B, INR L, DCR C, RAL and many more. Step 4 : (State T 4) In T 4, microprocessor decodes the opcode, and on the basis of the instruction received, it decides whether to enter state T 5 or to enter state T 1 of the next Machine Cycle of 8085 Microprocessor. Step 3 : (State T 3) During T 3, 8085 loads the data from the data bus in its Instruction Register and raises RD to high which disables the memory device. The memory device then places the contents of addressed memory location on the data bus (AD 0 – AD 7). In T 2, 8085 sends RD signal low to enable the addressed memory location. (However A 0 – A 7 remain available as they were latched during T 1). Step 2 : (State T 2) In T 2, low-order address disappears from the AD 0 – AD 7 lines. In opcode fetch machine cycle status signals are : IO/M = 0, S 1 = 1, S 0 = 1. IO/M specifies whether it is a memory or I/O operation, S 1 status specifies whether it is read/write operation S 1 and S 0 together indicates read, write, opcode fetch, machine cycle operation, or whether it is in HALT state. In T 1, 8085 also sends status signals IO/M, S 1, and S 0. Thus microprocessor activates ALE (Address Latch Enable) which is used to latch the low-order byte of the address in external latch before it disappears. The low-order byte of the PC is placed on the AD 0 – AD 7 lines which stays on only during T 1. The high-order byte of the PC is placed on the A 8 – A 15 lines. Step 1 : (State T 1): In T 1 state, the 8085 places the contents of program counter on the address bus. The following section describes the opcode fetch cycle in step by step manner. It varies from 4T states to 6T states as per the instruction. 1.15 (b) shows the timing diagram for Opcode Fetch Machine Cycle 8085. 1.15 (a) shows flow of data (opcode) from memory to the microprocessor and Fig. In this Machine Cycle in 8085, processor places the contents of the Program Counter on the address lines, and through the read process, reads the opcode of the instruction. The first Machine Cycle of 8085 Microprocessor of every instruction is opcode fetch cycle in which the 8085 finds the nature of the instruction to be executed. ![]()
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