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ECE291 Computer Engineering II Lecture 5

ECE291 Computer Engineering II Lecture 5. Josh Potts University of Illinois at Urbana- Champaign. Outline. Unconditional jump Conditional branching Construction of loops. Unconditional Jump JMP.

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ECE291 Computer Engineering II Lecture 5

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  1. ECE291Computer Engineering IILecture 5 Josh Potts University of Illinois at Urbana- Champaign

  2. Outline • Unconditional jump • Conditional branching • Construction of loops ECE291

  3. Unconditional JumpJMP • Short jump2-byte instruction that allows jumps or branches to memory locations within +127 and -128 bytes from the memory location following the jump JMP SHORT Label • Near jump3-byte instruction that allows jumps or branches within +/- 32Kb from the instruction in the current code segment JMP Label • Far jump5-byte instruction that allows a jump to any memory location with in the entire memory space JMP Label • For 80386, 80486, the near jump is within +/-2G if the machine operates in the protected mode and +/-32K bytes if operates in the real mode OPCODE DISP OPCODE DISP low DISP high OPCODE IP low IP high CS low CS high ECE291

  4. Conditional Branching • Logic and arithmetic instructions set flags • Flags provide state information from previous instruction(s) • Using flags we can perform conditional jumping, i.e., transfer program execution to some different place within the program if condition was true • jump back or forward in your code to the location specified • instruction pointer (IP) gets updated (to point to the instruction to which execution will jump) if condition was false • continue execution at the following instruction • IP gets incremented as usual ECE291

  5. Conditional Branching (cont.) • Conditional jumps are always short jumps in the 8086-80286 • the range of the jump is +127 bytes and -128 bytes from the location following the conditional jump • In 80386, 80486 conditional jumps are either short or near jumps • Conditional jumps test: sign (S), zero (Z), carry (C), parity (P), and overflow (O) • Note: an FFh is above the 00h in the set of unsigned numbers an FFh (-1) is less than 00h for signed numbers when you compare unsigned FFh is above 00h, but signed FFh is less than 00h ECE291

  6. Numerical Comparison • CMP(comparison) compares A to B • a subtraction that only changes the flag bits • useful for checking the entire contents of a register or a memory location against another value • usually followed by a conditional jump instruction CMP AL, 10h ;compare with 10h (contents of AL does not change) JAE SUBER ;if 10h or above then jump to memory location SUBER • SUB (subtraction) calculates difference A - B • saves results to A and set flags ECE291

  7. Numerical ComparisonCondition Code Settings CMP Oprnd1, Oprnd2 Unsigned Operands Signed operands Z: equality/inequality Z: equality/inequality C: Oprnd1 < Oprnd2 (C=1) C: no meaning Oprnd1 >= Oprnd2 (C=0) S: no meaning S and O taken together O: no meaning If ((S=0) and (O=1)) or ((S=1) and (O=0)) then Oprnd1 < Oprnd2 If ((S=0) and (O=0)) or ((S=1) and (O=1)) then Oprnd1 >= Oprnd2 ECE291

  8. Comparing Signed Integers • Consider CMP AX,BX computed by the CPU • The Sign bit (S) will be set if the result of AX-BX has a 1 at the most significant bit of the result (i.e., 15th bit for 16-bit op) • The Overflow flag (O) will be set if the result of AX-BX produced a number that was out of range (-32768 - 32767 for 16-bit numbers) to be represented by an integer. • Difference in JS (jump on sign) and JL (jump less than) • The conditional jump JS looks at the sign bit (S) of the last compare (or subtraction). If S = = 1 then jump. • The conditional jump JL looks (SXOR O) of the last compare (or subtraction) • REGARDLESS of the value AX-BX, i.e., even if AX-BX causes overflow, the JL will be correctly executed ECE291

  9. Comparing Signed Integers (cont.) • JL is true if the condition: S xor O is met • JL is true for two conditions: • S=1, O=0: • (AX-BX) was negative and (AX-BX) did not overflow • Example (8-bit): (-5) - (2) = (-7) Result (-7) has the sign bit set Thus (-5) is less than (2). ECE291

  10. Comparing Signed Integers (cont.) • S=0, O=1: • Overflow!,Sign bit of the result is wrong! • Consider the following case: AX is a large negative number (-) BX is a positive number (+). The subtraction of (-) and (+) is the same as the addition of (-) and (-) The result causes negative overflow, and thus cannot be represented as a signed integer correctly (O=1). The result of AX-BX appears positive (S=0). • Example (8-bit): (-128) - (1) = (+127) • Result (+127) overflowed. Answer should have been (-129). • Result appears positive, but overflow occurred • Thus (-128) is less than (1), i.e., the condition is TRUE for executing JL ECE291

  11. AX BX AX BX BX AX -8 -8 -8 -7 -7 -7 -6 -6 -6 -5 -5 -5 -4 -4 -4 -3 -3 -3 -2 -2 -2 -1 -1 -1 0 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 Comparing Signed IntegersCMP AX, BX AX – BX = 2 – (-4) = 2 + 4 = 6 =0110 So s = 0, no overflow (o = 0) Therefore AX >= BX AX – BX = 6 – (-3) = 6 + 3 = 9 = 1001 So s = 1, overflow (o = 1) Therefore AX >= BX AX – BX = 2 – 4 = -2 = 1110 So s = 1, no overflow (o = 0) Therefore AX < BX ECE291

  12. Conditional Branching (cont.) • Terminology used to differentiate between jump instructions that use the carry flag and the overflow flag • Above/Below unsigned compare • Greater/Less signed (+/-) compare • Names of jump instructions J => Jump N => Not A/B G/L => Above/Below Greater/Less E => Equal ECE291

  13. Summary of Conditional Jump Instructions Command Description Condition JA=JNBE Jump if above C=0 & Z=0 Jump if not below or equal JBE=JNA Jump if below or equal C=1 | Z=1 JAE=JNB=JNC Jump if above or equal C=0 Jump if not below Jump if no carry JB=JNAE=JC Jump if below C=1 Jump if carry JE=JZ Jump if equal Z=1 Jump if Zero JNE=JNZ Jump if not equal Z=0 Jump if not zero JS Jump Sign (MSB=1) S=1 ECE291

  14. Summary of Conditional Jump Instructions Command Description Condition JNS Jump Not Sign (MSB=0) S=0 JO Jump if overflow set O=1 JNO Jump if no overflow O=0 JG=JNLE Jump if greater Jump if not less or equal S=O & Z=0 JGE=JNL Jump if greater or equal S=O Jump if not less JL=JNGE Jump if less S^O Jump if not greater or equal JLE=JNG Jump if less or equal S^O | Z=1 Jump if not greater JCXZ Jump if register CX=zero CX=0 ECE291

  15. Mapping High Level Branches into Linear Code CMP AX, BX JA true_label …. <False Processing> …. JMP done_label …. true_label: <True processing> …. done_label: <resume execution> ECE291

  16. Mapping High Level Branches into Linear Code (cont.) ECE291

  17. Mapping High Level Branches into Linear Code (cont.) • LOOP instruction • combination of a decrement CX and a conditional jump • LOOP decrements CX (ECX if in 32-bit mode) and if CX  0 it jumps to the address indicated by the label • if CX becomes a 0, the next sequential instruction executes ADDS PROC NEAR MOV CX, 100 ; load count MOV SI, OFFSET BLOCK1 MOV DI, OFFSET BLOCK2 Again: LODSW ;get Block1 data; AX = [SI]; SI = SI + 2 ADD AX, ES:[DI] ;add Block2 data STOSW ;store in Block2; [DI] = AX; DI = DI + 2 LOOP Again ;repeat 100 times RET ADDS ENDP ECE291

  18. ; if (J <= K) then ; L := L + 1 ; else L := L - 1 ; J, K, L are signed words MOV AX, J CMP AX, K JNEL DoElse INC L JMP ifDone DoElse: DEC L ifDone: ; while (J >= K) do begin ; J := J - 1; ; K := K + 1; ; L := J * K; ; end; WhlLoop: MOV AX, J CMP AX, K JNGE QuitLoop DEC J INC K MOV AX, J IMUL AX, K MOV L, AX JMP WhlLoop QuitLoop: Examples ECE291

  19. Example (LOOPNE) • The LOOPNE instruction is useful for controlling loops that stop on some condition or when the loop exceeds some number of iterations • Consider String1 that contains a sequence of characters that end with the byte containing zero • we want to convert those characters to upper case and copy them to String2 ….. String1 BYTE “This string contains lower case characters”, 0 String2 BYTE 128 dup (0) …….. ECE291

  20. Example (LOOPNE) LEA SI, String1 ;the same as use of OFFSET LEA DI, String2 MOV CX, 127 ;Max 127 chars to String2 StrLoop: LODSB ;get char from String1; AL =[SI]; SI = SI + 1 CMP AL, ‘a’ ;see if lower case JB NotLower ;chars are unsigned CMP AL, ‘z’ JA NotLower AND AL, 5Fh ;convert lower -> upper case; ;bit 6 must be 0 NotLower: STOSB ; [DI] = AL; DI = DI + 1 CMP AL, 0 ;see if zero terminator LOOPNE StrLoop ;quit if AL or CX = 0 ECE291

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