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Architecture Instruction Set and Addressing mode
Microprocessor ?A microprocessor is multi programmable clock driven register based semiconductor device that is used to fetch , process and execute a data within fraction of seconds.
THE 8085 AND ITS The 8085 is BUSSES an 8-bit general purposemicroprocessor that can address 64K Byte of memory. It has 40 pins and uses +5V for power. It can run at a maximum frequency of 3 MHz. -The pins on the chip can be grouped into 6 groups: Address Bus. Data Bus. Control and Status Signals. Power supply and frequency. Externally Initiated Signals.
The address bus has 8 signal lines A8 A15 which are unidirectional. The other 8 address bits are multiplexed (time shared) with the 8 data bits. So, the bits AD0 AD7 are bi-directional and serve as A0 A7 and D0 D7 at the same time. During the execution of the instruction, these lines carry the address bits during the early part, then during the late parts of the execution, they carry the 8 data bits. In order to separate the address from the data, we can use a latch to save the value before the function of the bits changes.
The Address and Data Busses
8085 ARCHITECTUREINTA RST 5.5 RST 7.5 RST 6.5 INTR TRAP
Interrupt controller8 bit internal Data Bus
Serial I/O controller
Temp RegReg array
Flag Flip Flops
Instruction Decoder and Machine Cycle Encoding H L
Stack Pointer (16)Program counter (16)
Timing and controlX1 X2
Clk gen Address BufferControl Status DMA Reset
Data/ Address Buffer
A15 A8 Address Buss
AD0 AD7 Address/ Data Buss
Flag RegisterThe flags are affected by the arithmetic and logical instruction D7 S D6 Z D5 D4 AC D3 D2 P D1 D0 CY
AccumulatorIt is an 8 bit register For any arithmetic and logical instruction one of the data should be in this register It is used for storing the result of any arithmetic and logical manipulations. It is also called as A register All the data which are sent to I/O devices are sent via A register.
Temporary register It
is used to hold the data during the operation of arithmetic and logical operation
Sign Flag If
the D7 bit of the accumulator is set then this flag is set i.e 1 meaning that the result is in negative. Ex. 7-8 = -1
Carry flagDuring the arithmetic operation if a carry occurs then this flag is set. Ex. F1+1F= 1 10Carry
the arithmetic/ logical operation if the result is zero then this flag is set. Ex. FF-FF = 00
Parity flag After
the of the arithmetic and logical operation if the result is even then this flag is set. Ex. 0A-02 = 08
Auxiliary carry flag During
BCD arithmetic operation when a carry is generated by D3 bit and passed on to D4 bit then this flag is set. Ex. 1F+11 = 0001 1111 + 0001 0001 = 0010 0000
Timing and control It
synchronizes all the operation with the clock and generates the communication between the microprocessor and peripherals
Instruction Register and decoder The instruction is loaded in the instruction register The decoder decodes them and establishes the operation that has to be performed
Register array The
W and Z register are temporary registers Used to hold the 8 bit data during the execution and it is used internally . It is not used by the programmer.
Control and status signalsMachine Cycle Opcode fetch Memory read Memory write I/O read I/O write Interrupt ack Halt Hold IO/M 0 0 0 1 1 1 Z Z S1 1 1 0 1 0 1 0 X S0 1 0 1 0 1 1 0 X
Arithmetic and Logical unit It
is an 8 bit register It is used for performing addition, subtraction and logical operation. AND, OR, NOT, XOR, CMP are some of the logical operation.
is a 16 bit register It is used to point out the address of the next instruction which is to be executed
Stack pointer It
is a 16 bit register It points the starting address of the stack .
Register Array B,
C, D, E, H and L are general purpose register All are 8 bit register If the are combined as BC, DE and HL they can store 16 bit data
Instruction setAn instruction is a binary pattern designed inside a microprocessor to perform a specific function. A group of instruction together called as instruction set. Group of instruction set is called as a program.
Classification of instruction set
According to word size or byte size it is classified into 3 types. 1 - byte instruction 2 - byte instruction and 3 - byte instruction
1 byte instruction It
includes the Opcode and the Operand in the same byte. Ex. MOV A,B Ex. CMP B Ex. ANA B Ex. RAL
Opcode an operand The
task to be performed is called Opcode The data to be operated is called Operand.
2 byte instruction The
first byte specifies the operation cod eand the next byte specifies the operand Ex. MVI A, 10 Ex. SUI A, 34
3 byte instruction The
first byte specifies the opcode and the next two bytes specifies the 16 bit address/data. Ex. LXI H, 4500 Ex. JMP 5000
Classification of InstructionsThe 8085 instruction are classified into 5 categories. They are.. Data transfer operations Arithmetic operations Logical operations Branching operations and Machine control operation
Data transfer operationsGroup of data form a source location are copied to the destination location without changing the original data. Various types of data transfer are: Between registers Specific byte to a register or a memory location Between memory location and a register Between an I/O device and accumulator
Arithmetic operations Function
like addition, subtraction, increment and decrement operation are performed Ex. ADD B Ex. SUB B Ex. INR B Ex. DCX H
Logical operation These
instruction perform various logical operation with the contents of the accumulator. Ex. AND B Ex. ORA Ex. RAR Ex. CMP B Ex. CMA
Branching operation It
alters the sequence of program executing either conditionally are unconditionally Ex. JMP 5000 Ex. JNC 4500
Machine control operation It
controls the machine operation Ex. HALT Ex. NOP Ex. INTR
Addressing modes The
various ways of specifying the operand are called the addressing mode. It classified is as Immediate addressing Register addressing Direct addressing Indirect addressing Implied addressing mode
Immediate addressing modeInstructions that use immediate addressing
MVI R, DATA
Register Addressing mode
MOV Rd, Rs
Direct addressing modeInstructions that include a direct address IN/OUT Port # LDA 4500
Indirect addressing mode
Register indirect instructions reference memory via a register pair.
LXI Rp, 16 bit address
Implied addressing modeInstruction deals with the accumulatorORA RAL