CE302 MICROPROCESSORS Levent EREN Izmir University of Economics

CE302 Outline Unconditional jump Conditional branching Construction of loops

CE302 Unconditional Jump JMP Short jump 2-byte instruction that allows jumps or branches to memory locations within +127 and -128 bytes from the memory location following the jump JMPSHORT Label Near jump 3-byte instruction that allows jumps or branches within +/- 32Kb from the instruction in the current code segment JMPLabel Far jump 5-byte instruction that allows a jump to any memory location with in the entire memory space JMPLabel 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

CE302 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

CE302 Conditional Branching (cont.) Conditional jumps are always short jumps in the –the range of the jump is +127 bytes and -128 bytes from the location following the conditional jump In 80386, 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

CE302 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 CMPAL, 10h ;compare with 10h (contents of AL does not change) JAESUBER ;if 10h or above then jump to memory location SUBER SUB (subtraction) calculates difference A - B –saves results to A and set flags

CE302 Numerical Comparison Condition Code Settings CMPOprnd1, Oprnd2 Unsigned OperandsSigned operands 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 meaningIf ((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

CE302 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 ( 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 (S XOR 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

CE302 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).

CE302 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

CE302 Comparing Signed Integers CMP AX, BX AXBX AXBX AXBX AX – BX = 2 – (-4) = = 6 =0110 So s = 0, no overflow (o = 0) Therefore AX >= BX AX – BX = 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

CE302 Conditional Branching (cont.) Terminology used to differentiate between jump instructions that use the carry flag and the overflow flag –Above/Belowunsigned compare –Greater/Lesssigned (+/-) compare Names of jump instructions J => Jump N => Not A/B G/L=> Above/Below Greater/Less E=>Equal

CE302 Summary of Conditional Jump Instructions CommandDescriptionCondition JA=JNBEJump if above C=0 & Z=0 Jump if not below or equal JBE=JNAJump if below or equalC=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 equalZ=0 Jump if not zero JSJump Sign (MSB=1) S=1

CE302 Summary of Conditional Jump Instructions CommandDescriptionCondition JNSJump Not Sign (MSB=0) S=0 JO Jump if overflow set O=1 JNOJump if no overflow O=0 JG=JNLE Jump if greater Jump if not less or equal S=O & Z=0 JGE=JNLJump if greater or equal S=O Jump if not less JL=JNGEJump 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 JCXZJump if register CX=zero CX=0

CE302 Mapping High Level Branches into Linear Code CMP AX, BX JA true_label …. …. JMP done_label …. true_label: …. done_label:

CE302 Mapping High Level Branches into Linear Code (cont.)

CE302 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 ADDSPROC NEAR MOVCX, 100; load count MOVSI, OFFSET BLOCK1 MOVDI, OFFSET BLOCK2 Again: LODSW;get Block1 data; AX = [SI]; SI = SI + 2 ADDAX, ES:[DI];add Block2 data STOSW;store in Block2; [DI] = AX; DI = DI + 2 LOOPAgain;repeat 100 times RET ADDSENDP

CE302 Examples ;if (J <= K) then ;L := L + 1 ;elseL := L - 1 ; J, K, L are signed words MOVAX, J CMPAX, K JNELDoElse INCL JMPifDone DoElse: DECL ifDone: ;while (J >= K) do begin ;J := J - 1; ;K := K + 1; ;L := J * K; ;end; WhlLoop: MOVAX, J CMPAX, K JNGEQuitLoop DECJ INCK MOVAX, J IMULAX, K MOVL, AX JMPWhlLoop QuitLoop:

CE302 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 ….. String1BYTE“This string contains lower case characters”, 0 String2BYTE128 dup (0) ……..

CE302 Example (LOOPNE) LEASI, String1 ;the same as use of OFFSET LEADI, String2 MOVCX, 127;Max 127 chars to String2 StrLoop: LODSB;get char from String1; AL =[SI]; SI = SI + 1 CMPAL, ‘a’;see if lower case JBNotLower ;chars are unsigned CMPAL, ‘z’ JANotLower ANDAL, 5Fh;convert lower -> upper case; ;bit 6 must be 0 NotLower: STOSB; [DI] = AL; DI = DI + 1 CMPAL, 0;see if zero terminator LOOPNEStrLoop;quit if AL or CX = 0