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Flow Control

Flow Control. 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. ALU.

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Flow Control

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  1. Flow Control Flow Control 1

  2. 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 Flow Control 2

  3. ALU 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 won’t be using this bit. … N Z V C … Flow Control 3

  4. Table C.5 Flow Control 4

  5. Table C.4 Flow Control 5

  6. 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) Z=1 SUB.W D3,D4 BEQ LOOP • when does Z=1? • When [D3] and [D4] are equal! • Remember that CMP and SUB compute[dest] – [src] Flow Control 6

  7. CMP & Bcc instructions • CMP Operations performed: [d] – [s] • Result doesn’t go anywhere! Why? • Bcc represents many instructions: • BEQ – branch when equal (Z=1) • BNE – branch when not equal (Z=0) • BVS – branch when overflow set (V=1) • BPL – branch when result positive (N=0) • BRA – always branch • See table C.6 Flow Control 7

  8. 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 Flow Control 8

  9. 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 < 0 ARRAY DC.W 12,4,8 Flow Control 9

  10. Flow Control: for loops • for(i=0; i<5; 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… Flow Control 10

  11. Flow Control: Fixed loops MOVEI.B #5,D0 LOOP do something… SUBQ.B #1,D0 BNE LOOP ;BRA if Z=0 move on… • Using a down counter. A more efficient way to loop 5 times: Flow Control 11

  12. Flow Control: while loops • while (j < 5) {…} Test at beginning. • condition: (j < 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 Flow Control 12

  13. Flow Control: Other ways to use branch • You don’t 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… We’ll use D0 for j BRA if Z=1 BRA doesn’t affect the CCR Flow Control 13

  14. 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… Flow Control 14

  15. Flow Control: If…then…else • if (y > 3){ true-code}else{ 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. Flow Control 15

  16. 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 $1008 10 $100A 5 $100C 12 $100E 14 $1010 ? SUM Flow Control 16

  17. Branches and Overflow • In the 68000 the V bit is set on 2’s 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) • N=0 when D2  D1 • What if [D1] = 1, and [D2] = –128? • Can we represent –129 in an 8-bit byte in 2’s complement? (10000000 + 11111111) • The result is 127 (positive), N=0, V=1 • We don’t branch, which is good since –128 < 1 ! Flow Control 17

  18. Operation sizes and Overflow • In the 68000, the V-bit is set when there is a 2’s complement overflow for the size of operand specified in the instruction! • In other words, suppose D0 = $00000063 • ADD.B #$60,D0 sets V=1 and N=1 • ADD.W #$60,D0 sets V=0 and N=0 • 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. Flow Control 18

  19. 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 Flow Control 19

  20. 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 is:0110 1100 1111 0110= $6CF6 Flow Control 20

  21. Putting it together: Summing an array ORG $1000 CLR.W D3 ; The sum will go into D3 MOVEA.L #NUMS,A1 ; A1 -> 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 Flow Control 21

  22. Flow Control: In-Class Exercises Flow Control 22

  23. 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 (i=0; i<100; i++) { for (j=0; j<100; j++) { counter += 3; } } Flow Control 23

  24. 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 Flow Control 24

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