Overview The von Neumann Machine - the programmable digital computer Introducing the LC-3 Computer - A “toy” computer for us to learn from Computer machine instructions - the basic instructions that all programs use on that computer The Instruction Cycle - the steps in the execution of a machine language instruction

From Logic to Data Path The data path of a computer is all the logic used to process information. –See the data path of the LC-3 on next slide. Combinational Logic –Decoders -- convert instructions into control signals –Multiplexers -- select inputs and outputs –ALU (Arithmetic and Logic Unit) -- operations on data Sequential Logic –State machine -- coordinate control signals and data movement –Registers and latches -- storage elements

LC-3 Data Path: Combinational Logic State Machine Storage

The LC-3 as a von Neumann machine

Data Path of the LC-3

Computer Machine Instruction Formats What is IN an instruction? Specifies the operation to be performed Where to get the operands Where to put the result What are the major instruction types? Operate (add, sub, mult,OR, AND, etc.) Control (branch, jump to subroutine, etc.)

The Machine Instruction Necessary information: Operation code Input Operand(s) Output Operand(s) Typical Operation formats: 1 operand 2 operands 3 operands

The LC-3 Instruction FORMAT – generally 2 operand Operation code Input Operand(s) Output Operand(s) Where are the operands? Memory Registers

The LC-3 Instruction ADDRESSING MODES Register (Operand is in one of the 8 registers) PC-relative (Operand is “offset” from the (PC) ) Base + Offset (Base relative) (Operand is “offset” from the contents of a register) Immediate (Operand is in the instruction) Indirect (The “Operand” actually points to the real address – rather than being the operand)

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

LC-3 Memory Layout X000 – x2FFF Operating System programs, tables, and data Generally off limits to programmer (Prog run in Supervisor mode) X3000 – xFDFF User Program Area Area shared by users like you (Run in non-supervisor mode) xFE00 – xFFFF Device Register Addresses Pseudo memory used for input/output R0-R7 Registers