Instruction Execution: Straight-Line Sequencing and Branching PDF

Instruction Execution: Straight- line sequencing and branching  Instruction Execution and Straight-Line Sequencing Address Contents i Assu...

Instruction Execution: Straight- line sequencing and branching  Instruction Execution and Straight-Line Sequencing Address Contents i Assumptions: Begin execution here Move A,R0 3-instruction - One memory operand i+4 Add B,R0 program i+8 segment per instruction Move R0,C - 32-bit word length - Memory is byte addressable A - Full memory address can be directly specified in a single-word instruction B Data for the program Two-phase procedure -Instruction fetch -Instruction execute C A program for [C]   +   i Move NUM1,R0 Branching i+4 i+8 Add Add NUM2,R0 NUM3,R0 i + 4n - 4 Add NUMn,R0 i + 4n Move R0,SUM SUM NUM1 NUM2 NUMn A straight-line program for adding n numbers.  Move N,R1 Branching LOOP Clear R0 Determine address of "Next" number and add Program "Next" number to R0 loop Decrement R1 Branch target Branch>0 LOOP Move R0,SUM Conditional branch SUM N n NUM1 Using a loop to add n numbers. NUM2 NUMn  Condition Codes Condition code flags Condition code register / status register N (negative) Z (zero) V (overflow) C (carry) Different instructions affect different flags  Conditional Branch Instructions Example: A: 11110000 A: 1 1 1 1 0 0 0 0 +(−B): 1 1 1 0 1 1 0 0 B: 0 0 0 1 0 1 0 0 11011100 C=1 Z=0 N=1 V=0  Status Bits Cn-1 A B Cn ALU F V Z N C Fn-1 Zero Check Addressing Modes  Generating Memory Addresses How to specify the address of branch target? Can we give the memory operand address directly in a single Add instruction in the loop? Use a register to hold the address of NUM1; then increment by 4 on each pass through the loop.  Addressing Modes Opcode Mode... Implied – AC is implied in “ADD M[AR]” in “One-Address” instr. – TOS is implied in “ADD” in “Zero-Address” instr. Immediate – The use of a constant in “MOV R1, 5”, i.e. R1 ← 5 Register – Indicate which register holds the operand  Addressing Modes Register Indirect – Indicate the register that holds the number of the register that holds the operand R1 MOV R1, (R2) R2 = 3 Autoincrement / Autodecrement – Access & update in 1 instr. R3 = 5 Direct Address – Use the given address to access a memory location  Addressing Modes Indirect Address – Indicate the memory location that holds the address of the memory location that holds the data AR = 101 100 101 0 1 0 4 102 103 104 1 1 0 A  Addressing Modes Relative Address 0 – EA = PC + Relative Addr 1 PC = 2 2 + 100 AR = 100 101 102 1 1 0 A Could be Positive or 103 Negative 104 (2’s Complement)  Addressing Modes Indexed – EA = Index Register + Relative Addr Useful with XR = 2 “Autoincrement” or “Autodecrement” + 100 AR = 100 101 Could be Positive or Negative 102 1 1 0 A (2’s Complement) 103 104  Addressing Modes Base Register – EA = Base Register + Relative Addr Could be Positive or AR = 2 Negative (2’s Complement) + 100 0 0 0 5 BR = 100 101 0 0 1 2 102 0 0 0 A Usually points to 103 0 1 0 7 the beginning of 104 0 0 5 9 an array  Addressing Modes Name Assembler syntax Addressingfunction The different ways in which Immediate #Value Op erand = Value the location of Register Ri EA = Ri an operand is specified in an Absolute(Direct) LOC EA = LOC instruction are referred to as Indirect (Ri ) EA = [Ri ] addressing (LOC) EA = [LOC] modes. Index X(R i ) EA = [Ri ] + X Basewith index (Ri ,Rj ) EA = [Ri ] + [Rj ] Basewith index X(R i ,Rj ) EA = [Ri ] + [Rj ] + X and offset Relative X(PC) EA = [PC] + X Autoincrement (Ri )+ EA = [Ri ] ; Increment R i Autodecrement  (Ri ) Decrement R i ; EA = [Ri ]  Indexing and Arrays Index mode – the effective address of the operand is generated by adding a constant value to the contents of a register. Index register X(Ri): EA = X + [Ri] The constant X may be given either as an explicit number or as a symbolic name representing a numerical value. If X is shorter than a word, sign-extension is needed.  Indexing and Arrays In general, the Index mode facilitates access to an operand whose location is defined relative to a reference point within the data structure in which the operand appears. Several variations: (Ri, Rj): EA = [Ri] + [Rj] X(Ri, Rj): EA = X + [Ri] + [Rj]  Relative Addressing Relative mode – the effective address is determined by the Index mode using the program counter in place of the general-purpose register. X(PC) – note that X is a signed number Branch>0 LOOP This location is computed by specifying it as an offset from the current value of PC. Branch target may be either before or after the branch instruction, the offset is given as a singed num.