Overview von Neumann Model Components of a Computer Some Computer Organization Models The Computer Bus An Example Organization: The LC-3

The von Neumann Model The Instruction is the smallest piece of work specified in a program – The program cannot do just a piece of an instruction The Computer consists of 5 parts: – Processing unit (CPU) – Memory – Input – Output – Control Unit The computer program is contained in memory with the data – in the memory, they are indistinguishable

Basic Computer Components

Memory Computer memories are characterized by - The number of locations: 2 m Each location is identified by an address –The number of bits in a location: n Each location can contain data of computer instructions “partitions” of the memory are restricted to use for: –Operating System –User program(s) –Input & Output (I/O) addresses (“mapped” I/O) Alternatively, I/O can have a separate set of addresses –Etc.

Computer Main Memory

CPU The CPU has to be able to perform operations, e.g. –Read from memory –Write to memory –Decode Instructions –Compute (Arithmetic, Logic,..) –Etc. The CPU will have temporary storage: –Registers to store data temporarily –A register to hold the Instruction being executed –A register to point to the next instruction to be executed –A register to hold the present status of the active program –Etc.

Expanded Basic Computer Components

Control Unit The control unit is a finite state machine that controls the operation of the computer –Controls reading from memory –Controls writing to memory –Decodes instructions –Controls the execution of instructions –Controls I/O reads and writes –Controls the operation and timing of the System Bus –Etc.

I/O Devices I/O devices are external to the computer and connected through “interfaces”: –They provide data buffering –They provide control of the I/O devices –They provide for communication with the computer

System Bus The System Bus provides a conduit for transfer on data between: –The Memory –The I/O Device Interfaces –The CPU The System Bus is composed of: –Data Lines –Address Lines –Control Lines

System Bus Virtually all of the communication between the computer components, e.g. movement of instructions and data, is conducted over the System Bus

Example Computer Organization

LC-3 Computer Overview The LC-3 Computer 16 bit machine, word addressable, 64K or 65,536 locations Computer Machine Instructions – Computer “native” instructions The basic instructions that all programs use on that computer (The “atomic” unit of work done by a computer – see next slide) The Architecture (Organization) The hardware (state machine) that executes the instructions The Memory Holds the Operating System, the Program, support routines, data,.. The Instruction Cycle The steps in the execution of an instruction (machine language)

The Instruction Cycle Steps (or phases or states): Fetch Next Instruction from Memory (PC)  (points to) next instruction PC  ( PC) + 1 Decode Fetched Instruction Evaluate Address (es) (find where the data is) Fetch Operand (s) (get data) Execute Operation Store Result (if specified)

Computer Machine Instruction Formats What is IN an instruction? Operation code – what to do Input Operand(s) – where to get input operands (memory, registers) Output Operand(s) – Where to put results (memory, registers) What are the major instruction types? Data Movement (load, store, etc.) Operate (add, sub, mult, OR, AND, etc.) Control (branch, jump to subroutine, etc.)

What does an Instruction look like ? One Address Instruction: –Format: Op code + Address –The operation is performed on the Operand addressed and likely the value in an accumulator register in the CPU and likely places the result in the accumulator Two Address Instruction: –Format: Op code + Address + Address –The operation is performed on the 2 Operands addressed and places the result in one of the Operand Addresses Three Address Instruction: –Format: Op code + Address + Address + Address –The Operation is performed on two operand identified by two of the addresses and places the result where the third address specifies

More of Instruction Formats The Instruction contains the opcode and the operands or operand addresses Instructions are often limited to one word length If so, the opcode and all operands/addresses must fit in one word. Depending on the word length, this can severely limit the instruction format.

LC-3 Instructions (Fig 5.3 – Appendix a) Memory Move Instructions: LD, LDI, LDR ST, STI, STR LEA Arithmetic/Logic Instructions: ADD, AND, NOT Transfer/Control Instructions: BR, JMP TRAP, JSR, JSRR RET, RTI

Data Paths of the LC-3

Important Registers in the CPU 8 General Purpose Registers – Holds Data/Addresses PC – Points to the next instruction to be executed IR – holds the instruction being executed PSW (includes NZP) – holds the status of the program being executed MAR – Holds the address of the memory location being accessed MDR – Hold the data to be written into memory or the date read from memory

Components of the LC-3 CPU Combinational Logic State Machine Storage

LC-3 Memory Layout MAIN MEMORY: x0000 – x2FFF System: Operating System programs, tables, and data - Generally off limits to programmer (Programs run in Supervisor mode) x3000 – xFDFF User: User Program and Data Area Area shared by users like you (Nominally run in non-supervisor mode) xFE00 – xFFFF Device: I/O Register Addresses Pseudo memory used for input/output CPU MEMORY: R0-R7 Registers (16 bit)

LC-3 Memory Map

The LC-3 Instruction Addressing Modes Register (Operand is in one of the 8 registers) Immediate (Operand is in the instruction) PC-relative (Operand is “offset” from the (PC) ) Indirect (The “Operand” actually points to the real address of the Operand – rather than being the operand) Base + Offset (Base relative) (Operand is “offset” from the contents of a register) Note: NO Direct Addressing is defined in the LC-3 Direct Addressing (“Operand” is the address of the Operand)

The LC-3 Computer a von Neumann machine Memory PSW (Program Status Word): Bits: | S| |Priority| | N| Z| P| PSW The Instruction Cycle Fetch: Next Instruction from Memory (PC)  (points to) next instruction PC  (PC) + 1 Decode: Fetched Instruction Evaluate: Instr & Address (es) (find where the data is) Fetch: Operand (s) (get data as specified) Execute: Operation Store: Result (if specified)

LC-3 Instructions (Fig 5.3 – Appendix a) Addressing Modes Register (Operand is in one of the 8 registers) PC-relative (Operand is “offset” from where the PC points) Base + Offset (Base relative) (Operand is “offset” from the contents of a register) Immediate (Operand is in the instruction) Indirect (The “Operand” points to the real address of Operand – rather than being the operand)