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1. Architectures

2. What is Architecture? Computer Architecture – A set of rules and methods that describe the functionality, management and implementation of computers. To be precise, it is nothing but rules by which a system performs and operates. Can be divided into three main categories Instruction set Architecture or ISA − instruction is given to processor, read and act accordingly. It allocates memory to instructions and also acts upon memory address mode. Micro Architecture − It describes how a particular processor will handle and implement instructions from ISA. System design − includes other entire hardware component within the system such as virtualization, multiprocessing.

3. Instructions and instruction sequencing instruction sequencing is the order in which the instructions in a program are carried out. Normally the sequence proceeds in a linear fashion through the program, and the address of the instructions is obtained from the program counter in the control unit. This sequence is interrupted when a branch instruction is executed; at such a time the address field of the branch instruction is inserted into the program counter and the process continues. In the case of an indirect branch instruction, the memory content referred to by the address field of the instruction is inserted into the program counter.

4. Instructions Sequence Computer has instructions capable of performing four types of operations. Data transfers between the memory and the registers (MOV, PUSH, POP, XCHG). Arithmetic and logic operations on data (ADD, SUB, MUL, DIV, AND, OR, NOT). Program sequencing and control(CALL.RET, LOOP, INT). I/0 transfers (IN, OUT).

5. Register Transfer Notation ( RTN ) Generally info is transferred from one location to another in a computer and possible locations for such transfers are memory locations , processor registers , or registers in the I/O subsystem . E.g. Names for the addresses of memory locations may be LOC, PLACE, A, VAR2; Processor register names may be RO, R5; I/O register names may be DATAIN, OUTSTATUS, etc. Contents of any location are denoted by placing square brackets around its name.   R1 ← [LOC] Adding contents of registers R1 and R2, and placing their sum into register R3 can be highlighted as R3 ← [R1] + [R2] , THUS (RTN)

6. Assembly language Notation We can use assembly language another type of notation to represent machine instructions and programs. E.g. a generic instruction that causes the transfer described before, from memory location whose address is stored in LOC to processor register R1, is specified by the statement Move R1, [LOC]

7. Assembly and machine language Machine Language Assembly Language Machine language is only understand by the computers. Assembly language is only understand by human beings not by the computers. In machine language data only represented with the help of binary format(0s and 1s), hexadecimal and octadecimal. In assembly language data can be represented with the help of mnemonics such as Mov, Add, Sub, End etc. Machine language is very difficult to understand by the human beings. Assembly language is easy to understand by the human being as compare to machine language. Modifications and error fixing cannot be done in machine language. Modifications and error fixing can be done in assembly language. Machine language is very difficult to memorize so it is not possible to learn the machine language. Easy to memorize the assembly language because some alphabets and mnemonics are used. Execution is fast in machine language because all data is already present in binary format. Execution is slow as compared to machine language.

8. Assembly and machine language…. Machine Language Assembly Language There is no need of translator.The machine understandable form is the machine language. Assembler is used as translator to convert mnemonics into machine understandable form. Machine language is hardware dependent. Assembly language is the machine dependent and it is not portable.

9. Basic Instruction Types Three Address Instruction Add A, B, C Two Address Instruction Move B,C Add A,C  One Address Instruction Accumulator is used Add A - Add the contents of memory location A to the contents of accumulator register and place the sum back to the  accumulator .

10. Instruction Execution Straight-Line Sequencing - this is when the processor control circuits use the information in the PC to fetch and execute instructions, one at a time, in the order of increasing addresses. Branching - This type of instruction loads a new address into the program counter. As a result, the processor fetches and executes the instruction at this new address, called the branch target instead of following sequential address order .

11. The von Neumann Architecture IAS Architecture Mathematician John von Neumann and his colleagues proposed the von Neumann architecture in 1945, which stated that a computer consists of: a processor with an arithmetic and logic unit (ALU) and a control unit; a memory unit that can communicate directly with the processor using connections called buses; connections for input/output devices; and a secondary storage for saving and backing up data.

12. Von Neumann Architecture…

13. The Harvard Architecture This architecture keeps instructions and data in separate memories, The processor accesses these memories using separate buses and is connected to the ‘instructions memory’ using a dedicated set of address and data buses, and is connected to the ‘data memory’ using a different set of address and data buses. This architecture is used extensively in embedded computing systems such as digital signal processing (DSP) systems, and many microcontroller devices use a Harvard-like architecture.

14. The Harvard Architecture…

15. SIMPLIFIED

16. Comparisons  The major difference between the two architectures is that in a Von Neumann architecture all memory is capable of storing all program elements, data and instructions; in a Harvard architecture the memory is divided into two memories, one for data and one for instructions. Major issue involved in deciding which architecture to use is that some operations have to access memory both to fetch the instruction to execute, and to access data to operate on. Since m emory can only be accessed once per clock cycle, in principal a Von Neumann architecture requires at least two clock cycles to execute an instruction, whereas a Harvard architecture can execute instructions in a single cycle.

17. VON NEUMANN ARCHITECTURE HARVARD ARCHITECTURE It is ancient computer architecture based on stored program computer concept. It is modern computer architecture based on Harvard Mark I relay based model. Same physical memory address is used for instructions and data. Separate physical memory address is used for instructions and data. There is common bus for data and instruction transfer. Separate buses are used for transferring data and instruction. Two clock cycles are required to execute single instruction. An instruction is executed in a single cycle. It is cheaper in cost. It is costly than Von Neumann Architecture. CPU can not access instructions and read/write at the same time. CPU can access instructions and read/write at the same time. It is used in personal computers and small computers. It is used in micro controllers and signal processing.

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