Flow Control

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Flow Control
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Flow Control Goals

• Branching
• Condition code register
• Branch instructions
• Conditional branches
• Unconditional branches
• Know how to implement
• For loops
• While loops
• if/then/else statements
• Assigned Reading
• HVZ 3.11 Program Flow Control

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Flow Control The Condition Code Register

• The Condition Code Register (CCR) holds
  information about the result of the most recently
  executed arithmetic instruction.
• There are five bits that are set by the ALU.
• N 1 if the last result was Negative
• Z 1 if the last result was Zero
• V 1 if the last operation resulted in
  arithmetic overflow
• C 1 if the last operation produced a carry
• addition 1 if there was a carry out from most
  significant bit
• subtraction 1 if there was a borrow required
• X special operand for multiprecision
  computations. We wont be using this bit.

N Z V C
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Table C.5
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Table C.4
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Flow Control Branch Instructions

• The mnemonics for the branch instructions assume
  that you are following a SUB or a CMP
  instruction
• BEQ (branch when equal) Z1
• SUB.W D3,D4
• BEQ LOOP
• when does Z1?
• When D3 and D4 are equal!
• Remember that CMP and SUB computedest src

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CMP Bcc instructions

• CMP Operations performed d s
• Result doesnt go anywhere! Why?
• Bcc represents many instructions
• BEQ branch when equal (Z1)
• BNE branch when not equal (Z0)
• BVS branch when overflow set (V1)
• BPL branch when result positive (N0)
• BRA always branch
• See table C.6

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Flow Control Conditional Branch Instructions -
Bcc

• CMP src, dest
• BEQ Branch to TARGET if src dest
• BNE Branch to TARGET if src ! dest
• BLT Branch to TARGET if dest is less than src
• BLE Branch to TARGET if dest is less than or
  equal to src
• BGT Branch to TARGET if dest is greater than
  src
• BGE Branch to TARGET if dest is greater than or
  equal to src

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Flow Control Bcc

• You can also think of Bcc as comparing the result
  of the last operation to zero

MOVE.W -3,D0 D0 is a counter,
starting LEA ARRAY,A0 at the value
-12 LOOP ADD.W (A0),D1 ADDQ.W 1,D0 Add 1 to
the counter BLT LOOP Loop while result lt
0 ARRAY DC.W 12,4,8
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Flow Control for loops

• for(i0 ilt5 i)
• CLR.B D0
• LOOP
• ADDQ.B 1,D0
• CMPI.B 5,D0
• BLT LOOP

This is easy to read, and necessary if you want
to use the value of i.
D0 - 5
However, you have to get the immediate value 5
from memory repeatedly. There is a more
efficient way to loop 5 times
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Flow Control Fixed loops

• Using a down counter. A more efficient way to
  loop 5 times

• MOVEI.B 5,D0
• LOOP do something
• SUBQ.B 1,D0
• BNE LOOP BRA if Z0
• move on

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Flow Control while loops

• while (j lt 5) Test at beginning.
• condition (j lt 5) opposite (j ? 5)
• MOVE.W j,D0 get j from memory
• LOOP CMPI.W 5,D0
• BGE NEXT exit loop if
• condition false
• ADDQ 1,D0
• BRA LOOP
• NEXT
• j DC.W 2

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Flow Control Other ways to use branch

• You dont have to follow the mnemonics
• The best thing to do is to look at the branch
  condition in Table C.6
• EXAMPLE for(j-5 j!0 j)
• MOVE.B -5,D0
• LOOP BEQ DONE
• do something
• ADDQ.B 1,D0
• BRA LOOP
• DONE move on

Well use D0 for j
BRA if Z1
BRA doesnt affect the CCR
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Flow Control Conditionals

• if (x 5)
• The most efficient way to code this is to skip
  the code if the condition is not true.
• MOVE.W x,D2
• CMPI.W 5,D2
• BNE SKIP
• SKIP next instruction

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Flow Control Ifthenelse

• if (y gt 3) true-codeelse false-code

MOVE.W Y,D0 CMPI.W 3,D0 BLE ELSE true-code
BRA NEXT ELSE false-code NEXT next-instruction
Again we test for the opposite of the if
condition, and skip the true-code if necessary.
At the end of the true-code, we use a BRA to
avoid also executing the false code.
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Example Adding n integers
MOVE.W N,D1 MOVE.W NUM,A2 CLR.W
D0 LOOP ADD.W (A2),D0 SUB.W 1,D1 BGT
LOOP MOVE.W D0,SUM END ORG 001000 N DC.W
7 NUM DC.W 3,5,8,10,5,12,14 SUM DS 1
MEMORY 1000 7 N 1002 3 NUM 1004 5 1006 8 10
08 10 100A 5 100C 12 100E 14 1010 ? SUM
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Branches and Overflow

• In the 68000 the V bit is set on 2s complement
  overflow for the operand size (B, W, L)
• BGE (branch when greater or equal)
• Branch when N?V 0
• Example SUB.B D1, D2 (DEST SRC)
• N0 when D2 ? D1
• What if D1 1, and D2 128?
• Can we represent 129 in an 8-bit byte in 2s
  complement? (10000000 11111111)
• The result is 127 (positive), N0, V1
• We dont branch, which is good since 128 lt 1 !

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Operation sizes and Overflow

• In the 68000, the V-bit is set when there is a
  2s complement overflow for the size of operand
  specified in the instruction!
• In other words, suppose D0 00000063
• ADD.B 60,D0 sets V1 and N1
• ADD.W 60,D0 sets V0 and N0
• Same thing goes for the carry bit
• If a byte operation would produce a carry into
  bit 8, the C bit is set, and bit 8 retains its
  old value.

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Machine Code Branches

• 0110 CCCC PPPP PPPP or0110 CCCC 0000 0000PPPP
  PPPP PPPP PPPP
• Displacement can be an 8-bit or 16-bit value.
• Determined by assembler
• Dependent on the size of the jump

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Machine Code Branches - example

• LOOP ADD.W D2,D3 1 word
• SUB.W NUM, D1 3 words
• BGE LOOP 1 word
• Assuming LOOP is at address 1000, then the PC
  contains 1008 after fetching the BGE instruction,
  so the offset is 10 or F6
• The entire instruction is0110 1100 1111 0110
  6CF6

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Putting it together Summing an array
ORG 1000 CLR.W D3
The sum will go into D3 MOVEA.L
NUMS,A1 A1 -gt current array element
MOVE.W LEN,D2 D2 number of array
elements remaining LOOP ADD.W (A1),D3
Add the next element SUBQ.W 1,D2
Now there is one less remaining
BNE LOOP Continue if D2 ! 0
MOVE.B EXIT,D7 TRAP
14 Exit back to the
simulator EXIT EQU 228
LEN DC.W 5 LEN
Size of the array NUMS DC.W
123,-56,453,-1045,765 NUMS the array
End
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Flow Control In-Class Exercises
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Did you know?

• You can give C a hint about which variables to
  keep in registers?

register int counter int i, j counter 0 for
(i0 ilt100 i) for (j0 jlt100 j)
counter 3
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Flow Control You Should Know

• Review
• Condition code register
• LEA instructions
• Branch instructions
• Know how to implement
• For loops
• While loops
• if/then/else statements
• Unconditional branches
• Next Topic
• The Stack and Subroutines
• HVZ 3.13 Stacks and Subroutines