8085

Microcomputer Major components of the computer - the processor, the control unit, one or more memory ICs, one or more I/O ICs, and the clock Major components of the computer - the processor, the control unit, one or more memory ICs, one or more I/O ICs, and the clock A single printed circuit board usually connects the ICs, making a computer called a microcomputer A single printed circuit board usually connects the ICs, making a computer called a microcomputer

Microcomputer

Basic Architecture Princeton and Harvard Princeton and Harvard Princeton Architecture* Princeton Architecture* Known as Von Neumann architecture Known as Von Neumann architecture Single memory contains both the program code and the data. Single memory contains both the program code and the data. Harvard Architecture Harvard Architecture Two separate memories. One contains only data while the other is containing only program code. Two separate memories. One contains only data while the other is containing only program code.

Basic Architecture Princeton Harvard

Major components 1. Processor 2. Control Unit 3. Memory 4. Clock

Contents  Microprocessor Architecture Microprocessor Architecture Microprocessor Architecture

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.

PIN CONFIGURATION

THE 8085 AND ITS BUSSES  The 8085 is an 8-bit general purpose microprocessor 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.  Serial I/O ports.

The Address and Data Busses  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.

Interrupt controllerSerial I/O controller Accumulator ALU Flag Flip Flops Temp Reg Instruction Reg Instruction Decoder and Machine Cycle Encoding Multiplexer W temp Z temp BC D E HL Stack Pointer (16)Stack Pointer Program counter Program counter (16) Inc/dec latch 8085 ARCHITECTURE Timing and control Clk gen Control Status DMA ResetStatus Address Buffer Data/ Address Buffer Clk out Ready S0S0 S1S1 IO/M HOLD HLDA Reset in Reset out RD WR ALE X1 X2 A 15 –A 8 Address Buss AD 0 –AD 7 Address/ Data Buss SID SOD INTA RST 5.5RST 6.5RST 7.5 TRAP INTR 8 bit internal Data Bus Reg array

Flag Register CY PPPP AC ZZZZ SSSSD0D1D2D3D4D5D6D7 The flags are affected by the arithmetic and logical instruction

Accumulator  It 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 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 flag  During the arithmetic operation if a carry occurs then this flag is set.  Ex. F1+1F= 10 1 Carry

Zero flag  During 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 = = =

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 signals Machine Cycle IO/M S1S1S1S1 S0S0S0S0 Opcode fetch 011 Memory read 010 Memory write 001 I/O read 110 I/O write 101 Interrupt ack 111 HaltZ00 HoldZXX ResetZXX

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.

Program Counter  It 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