MICROPROCESSOR

INTERNAL STRUCTURE & BASIC OPERATION

1. ARITHMETIC LOGIC UNIT

ALU units typically need to be able to perform the basic logical operations (AND, OR) and the addition operation.

The inclusion of inverters on the inputs enables the same ALU hardware to perform the subtraction operation (adding an inverted operand), and the operations NAND and NOR.

EXAMPLE OF ALU ( 2 BITS )

This is an example of a basic 2-bit ALU. The boxes on the right hand side of the image are multiplexers and are used to select between various operations: OR, AND, XOR, and addition.

EXAMPLE OF ALU ( 4 BIT )

2. CONTROL UNIT

The Control Unit is one of the most important parts of the microprocessor because it is in charge of the entire process.

Creates control signals that tell the Arithmetic Logic Unit (ALU) and the Registers how to operate, what to operate on, and what to do with the result.

 The Control Unit makes sure everything happens in the right place at the right time.

3. REGISTER SET

Use to complete the tasks the Control Unit has requested

The data can come from the data cache, main memory, or the control unit, and are all stored at special locations within the Registers. 

4. ACCUMULATOR

The accumulator stores the result of every ALU operation, and is also one of the operands to every instruction. This means that our ISA can be less complicated, because instructions only need to specify one operand, instead of two operands and a destination

5. CONDITION CODE REGISTER

The code register is a hardware register which contains information about the state of the processor. Individual bits are implicitly or explicitly read and/or written by the machine code instructions executing on the processor

COMMON FLAG This is a list of the most common CPU status register flags,

implemented in almost all modern processors.Flag Name Description

Z Zero flag Indicates that the result of an arithmetic or logical operation (or, sometimes, a load) was zero.

C Carry flag Enables numbers larger than a single word to be added/subtracted by carrying a binary digit from a less significant word to the least significant bit of a more significant word as needed. It is also used to extend bit shifts and rotates in a similar manner on many processors (sometimes done via a dedicated X flag).

S / N Sign flagNegative flag

Indicates that the result of a mathematical operation is negative. In some processors,[the N and S flags are distinct with different meanings and usage: One indicates whether the last result was negative whereas the other indicates whether a subtraction or addition has taken place.

V / O / W

Overflow flag Indicates that the signed result of an operation is too large to fit in the register width using twos complement representation.

COMMON FLAG

This is a partial list of other, less common status flags that various CPUs support.

Flag Name Description

H / A Half-carry flagAuxiliary flagDecimal adjust flag

Indicates that a bit carry was produced between the nibbles (typically between the 4-bit halves of a byte operand) as a result of the last arithmetic operation. Such a flag is generally useful for implementing BCD arithmetic operations on binary hardware.

P Parity flag Indicates whether the number of set bits of the last result is odd or even.

I Interrupt flag Indicates that an interrupt is currently active, and that the currently executing code is part of an interrupt handler routine.

S Supervisor flag Indicates that CPU is operating in supervisor mode, having enhanced permissions enabled and access to special instructions typically reserved for the operating system itself, as opposed to the normal "user" instruction set.

6. PROGRAM COUNTER

The Program Counter (PC) is a register structure that contains the address pointer value of the current instruction. Each cycle, the value at the pointer is read into the instruction decoder and the program counter is updated to point to the next instruction

EXAMPLES

MIPSThe MIPS architecture uses a byte-addressable instruction memory unit. MIPS is a RISC computer, and that means that all the instructions are the same length: 32-bits. Every cycle, therefore, the PC needs to be incremented by 4 (32 bits = 4 bytes).

Intel IA32The Intel IA32 (better known by some as "x86") is a CISC architecture, which means that each instruction can be a different length. The Intel memory is byte-addressable. Each cycle the instruction decoder needs to determine the length of the instruction, in bytes, and it needs to output that value to the PC. The PC unit increments itself by the value received from the instruction decoder.

BRANCHING

Non Offset Branching- A non-offset branch, frequently referred to as a "jump" is a branch where the previous PC value is discarded and a new PC value is loaded from an external source.

BRANCHING

Offset Branching- An offset branch is a branch where a value is added (or subtracted) to the current PC value to produce the new value. This is typically used in systems where the PC value is larger then a register value or an immediate value, and it is not possible to load a complete value into the PC

7. STACK POINTER

A stack pointer is a small register that stores the address of the last program request in a stack. 

A stack (also called a pushdown stack) operates in a last-in/first-out sense. When a new data item is entered or "pushed" onto the top of a stack, the stack pointer increments to the next physical memory address, and the new item is copied to that address

BUS SYSTEM

Data Bus Address Bus Control Bus

DATA BUS

it is used to transfer data within Microprocessor and Memory/Input or Output devices.

It is bidirectional as Microprocessor requires to send or receive data.

The data bus also works as address bus when multiplexed with lower order address bus. Data bus is 8 Bits long.

ADDRESS BUS

used to transfer the addresses of Memory or I/O devices.

It is unidirectional.In Intel 8085 microprocessor, Address bus was of 16 bits.

This means that Microprocessor 8085 can transfer maximum 16 bit address which means it can address 65,536 different memory locations.

This bus is multiplexed with 8 bit data bus

CONTROL BUS

to process data, that is what to do with the selected memory location.

Some control signals are Read, Write and Opcode fetch etc. Various operations are performed by microprocessor with the help of control bus.

This is  a dedicated bus, because all timing signals are generated according to control signal.

CLOCK SYSTEM

The 8085 incorporates all of the features that the 8224 clock generator and 8228 system controller provided for the 8080A

X1 and X2: Are connected to a crystal, LC or RC network to drive the internal clock generator. The input frequency is divided by 2 to give the processor's internal operating frequency.

CLK: Clock output for use as a system clock. The period of CLK is twice the X1, X2 input period.

8085 clock system

INTEL 8085 MICROPROCESSOR FAMILY

Previous Generation   Related Family Next

Generation

•8080» 8-bit microprocessor•» Up to 4 MHz•» 64 KB RAM•» Stack in RAM•» 256 I/O ports40-pin DIP

•8085» 8-bit microprocessor•» Up to 8 MHz•» 64 KB RAM•» Single voltage•» On-chip peripherals•» 256 I/O ports•» 8080 object-code compatible40-pin DIP44-pin PLCC

•Z80» Up to 33 MHz•» 8080 emulation mode

•8086» 16-bit microprocessor•» 16-bit data bus•» Up to 10 MHz•» 1 MB RAM•» 64K I/O ports40-pin DIP56-pin QFP44-pin PLCC

CLOCK SYSTEM

  

6008 clock system

It comes in a 40-pin package that requires additional elements to make up a useful circuit. The normal clock signal speed is 1MHz, though the 6800 does not have an internal clock, ROM or RAM.

The 6800 has an 8-bit word size for instructions and data

The 6800 requires a two phase clock to operate.

The address bus is 16-bit allowing the microprocessor to access 64k 8-bit words in memory (Locations from 0000 to FFFF).

INTEL 6008 MICROPROCESSOR FAMILY

6802

The 6802 contains the 6800 CPU with the addition of 128 bytes of RAM for real time scratch pad operations.

The RAM is given the addresses 0000 to 007F so direct addressing can be used on it. There are 2048 bytes of ROM for program storage and an on-board clock. The 6802 uses the same instruction set as he 6800. 

INTEL 6008 MICROPROCESSOR FAMILY

6805The 6805 was a cheaper version of the 6803. It dropped some of the less used instructions to simplify operation. 6809The 6809 extended the 6800 capability into 16-bit applications, but it wasn't a true 16-bit microprocessor. The additional functionality was achieved by including extra registers, addressing facilities and instructions. The internal data buses were expanded to 16-bits