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2 .5- 2 Instruction Set Architecture ISA = All of the programmer-visible components and operations of the computer memory organization address space -- how may locations can be addressed? addressibility -- how many bits per location? register set how many? what size? how are they used? instruction set opcodes data types addressing modes ISA provides all information needed for someone that wants to write a program in machine language (or translate from a high-level language to machine language).
3 .5- 3 LC-3 Overview: Memory and Registers Memory address space: 2 16 locations (16-bit addresses) addressability: 16 bits Registers temporary storage, accessed in a single machine cycle accessing memory generally takes longer than a single cycle eight general-purpose registers: R0 - R7 each 16 bits wide how many bits to uniquely identify a register? other registers not directly addressible, but used by (and affected by) instructions PC (program counter), condition codes
4 .5- 4 LC-3 Overview: Instruction Set Opcodes 15 opcodes Operate instructions: ADD, AND, NOT Data movement instructions: LD, LDI, LDR, LEA, ST, STR, STI Control instructions: BR, JSR/JSRR, JMP, RTI, TRAP some opcodes set/clear condition codes , based on result: N = negative, Z = zero, P = positive (> 0) Data Types 16-bit 2’s complement integer Addressing Modes How is the location of an operand specified? non-memory addresses: immediate , register memory addresses: PC-relative , indirect , base+offset
5 .5- 5 Operate Instructions Only three operations: ADD, AND, NOT Source and destination operands are registers These instructions do not reference memory. ADD and AND can use “immediate” mode, where one operand is hard-wired into the instruction. Will show dataflow diagram with each instruction. illustrates when and where data moves to accomplish the desired operation
6 .5- 6 NOT (Register) Note: Src and Dst could be the same register.
7 .5- 7 ADD/AND (Register) this zero means “register mode”
8 .5- 8 ADD/AND (Immediate) this one means “immediate mode” Note: Immediate field is sign-extended .
9 .5- 9 LC-3 Data Path Revisited Filled arrow = info to be processed. Unfilled arrow = control signal.
10 .5- 10 Using Operate Instructions With only ADD, AND, NOT… How do we subtract? How do we OR? How do we copy from one register to another? How do we initialize a register to zero?
11 .5- 11 Control Instructions Used to alter the sequence of instructions (by changing the Program Counter) Conditional Branch branch is taken if a specified condition is true signed offset is added to PC to yield new PC else, the branch is not taken PC is not changed, points to the next sequential instruction Unconditional Branch (or Jump) always changes the PC TRAP changes PC to the address of an OS “service routine” routine will return control to the next instruction (after TRAP)
12 .5- 12 Condition Codes LC-3 has three condition code registers: N -- negative Z -- zero P -- positive (greater than zero) Set by any instruction that writes a value to a register (ADD, AND, NOT, LD, LDR, LDI, LEA) Exactly one will be set at all times Based on the last instruction that altered a register
13 .5- 13 Branch Instruction Branch specifies one or more condition codes. If the set bit is specified, the branch is taken. PC-relative addressing: target address is made by adding signed offset (IR[8:0]) to current PC. Note: PC has already been incremented by FETCH stage. Note: Target must be within 256 words of BR instruction. If the branch is not taken, the next sequential instruction is executed.
14 .5- 14 BR (PC-Relative) What happens if bits [11:9] are all zero? All one?
15 .5- 15 LC-3 Data Path Revisited Filled arrow = info to be processed. Unfilled arrow = control signal.
16 .How are Branches Used? Alter the flow of the program, based on some programmer-specified condition, evaluated at execution time. Two basic constructs: Iterative (Loops) execute instruction sequence multiple times Conditional (If, If-Else) choose whether or not to execute instruction sequence 5- 16
17 .Iterative 5- 17 While Loop Do-While Loop
18 .6- 18 Code for Iteration Exact bits depend on condition being tested PC offset to address C PC offset to address A Unconditional branch to retest condition Assumes all addresses are close enough that PC-relative branch can be used.
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