Lec3 instructions branch carl hamcher
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Transcript of Lec3 instructions branch carl hamcher
Instruction Execution: Straight-line sequencing and branching
Instruction Execution and Straight-Line Sequencing
R0,C
B,R0
A,R0
Movei + 8
Begin execution here Movei
ContentsAddress
C
B
A
the programData for
segmentprogram3-instruction
Addi + 4
A program for C +
Assumptions:- One memory operand per instruction- 32-bit word length- Memory is byte addressable- Full memory address can be directly specified in a single-word instruction
Two-phase procedure-Instruction fetch-Instruction execute
Branching
NUMn
NUM2
NUM1
R0,SUM
NUMn ,R0
NUM3,R0
NUM2,R0
NUM1,R0
A straight-line program for adding n numbers.
Add
Add
Move
SUM
i
Move
Add
i 4n+
i 4n 4-+
i 8+
i 4+
•••
•••
•••
BranchingN,R1Move
NUMn
NUM2
NUM1
R0,SUM
R1
"Next" number to R0
Using a loop to add n numbers.
LOOP
Decrement
Move
LOOP
loopProgram
Determine address of"Next" number and add
N
SUM
n
R0Clear
Branch>0
•••
•••
Branch target
Conditional branch
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: 1 1 1 1 0 0 0 0B: 0 0 0 1 0 1 0 0
A: 1 1 1 1 0 0 0 0
+(−B): 1 1 1 0 1 1 0 0
1 1 0 1 1 1 0 0
C = 1
N = 1
V = 0
Z = 0
Status Bits
ALU
V Z N C
Zero Check
Cn
Cn-1
Fn-1
A B
F
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
• Immediate• Direct• Indirect• Register• Register Indirect• Displacement (Indexed) • Implicit
Immediate Addressing
• Operand is part of instruction• Operand = address field• e.g. ADD 5
– Add 5 to contents of accumulator– 5 is operand
• No memory reference to fetch data• Fast• Limited range
Immediate Addressing Diagram
OperandOpcode
Instruction
Direct Addressing
• Address field contains address of operand• Effective address (EA) = address field (A)• e.g. ADD A
– Add contents of cell A to accumulator– Look in memory at address A for operand
• Single memory reference to access data• No additional calculations to work out effective
address• Limited address space
Direct Addressing Diagram
Address AOpcode
Instruction
Memory
Operand
Indirect Addressing (1)
• Memory cell pointed to by address field contains the address of (pointer to) the operand
• EA = (A)– Look in A, find address (A) and look there for
operand• e.g. ADD (A)
– Add contents of cell pointed to by contents of A to accumulator
Indirect Addressing (2)
• Large address space • 2n where n = word length• May be nested, multilevel, cascaded
– e.g. EA = (((A)))• Draw the diagram yourself
• Multiple memory accesses to find operand• Hence slower
Indirect Addressing Diagram
Address AOpcode
Instruction
Memory
Operand
Pointer to operand
Register Addressing (1)
• Operand is held in register named in address filed
• EA = R• Limited number of registers• Very small address field needed
– Shorter instructions– Faster instruction fetch
Register Addressing (2)
• No memory access• Very fast execution• Very limited address space• Multiple registers helps performance
– Requires good assembly programming or compiler writing
– N.B. C programming • register int a;
• c.f. Direct addressing
Register Addressing Diagram
Register Address ROpcode
Instruction
Registers
Operand
Register Indirect Addressing
• C.f. indirect addressing• EA = (R)• Operand is in memory cell pointed to by
contents of register R• Large address space (2n)• One fewer memory access than indirect
addressing
Register Indirect Addressing Diagram
Register Address ROpcode
Instruction
Memory
OperandPointer to Operand
Registers
Displacement Addressing
• EA = A + (R)• Address field hold two values
– A = base value– R = register that holds displacement– or vice versa
Displacement Addressing Diagram
Register ROpcode
Instruction
Memory
OperandPointer to Operand
Registers
Address A
+
Relative Addressing
• A version of displacement addressing• R = Program counter, PC• EA = A + (PC)• i.e. get operand from A cells from current
location pointed to by PC• c.f locality of reference & cache usage
Base-Register Addressing
• A holds displacement• R holds pointer to base address• R may be explicit or implicit• e.g. segment registers in 80x86
Indexed Addressing
• A = base• R = displacement• EA = A + (R)• Good for accessing arrays
– EA = A + (R)– R++
Combinations
• Postindex• EA = (A) + (R)
• Preindex• EA = (A+(R))
• (Draw the diagrams)
Implicit Addressing
• Operand is (implicitly) on top of stack• e.g.
– ADD Pop top two items from stackand add
Addressing Modes• The different
ways in which the location of an operand is specified in an instruction are referred to as addressing modes.
Name Assembler syntax Addressingfunction
Immediate #Value Operand=ValueRegister R i EA= RiAbsolute(Direct) LOC EA= LOCIndirect (Ri ) EA= [Ri]
(LOC) EA= [LOC]Index X(Ri) EA= [Ri]+ XBasewithindex (Ri ,Rj ) EA= [Ri]+ [Rj]Basewithindex X(Ri,Rj ) EA= [Ri]+ [Rj] + Xandoffset
Relative X(PC) EA= [PC] + XAutoincrement (Ri)+ EA= [Ri] ;
Increment RiAutodecrement (Ri ) Decrement R i ;
EA = [Ri]
BranchingN,R1Move
NUMn
NUM2
NUM1
R0,SUM
R1
"Next" number to R0
Using a loop to add n numbers.
LOOP
Decrement
Move
LOOP
loopProgram
Determine address of"Next" number and add
N
SUM
n
R0Clear
Branch>0
•••
•••
Branch target
Conditional branch
Utility of using Indirect Addressing
Using Auto-increment Mode
Utility of Indexed Addressing
A list of students’ marks
Find sum of Test1, Test2 and Test 3 scores