4.2.2 B X_BE ( BX i s B elow o r E qual) is a symbolic location JNBE / JA instruction if Not (Below or Equal) or if Above No Ex. Cy = 0 AND Z= 0

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1 4.2 Terms used in comparison Above and Below used for comparing Unsig ned numbers. Greater than and less than used when c omparing signed numbers. All Intel microprocessors use this conventio n. Accordingly, all the following statements are true. 95H is above 65H Unsigned comparison - True 95H is less than 65H S igned comparison T rue (as 95H is negative, 65H is positive) 65H is below 95H Unsigned comparison - Tr u e 65H is greater than 95H Signed comparison - T rue based on multiple f l ags Conditional s based on multiple flags are used after a CMP (compare) instructio n. JBE / JNA instr u ction Ju mp if Below or Equal or if Not Above No Ex. C y = 1 OR Z= 1 Cy = 0 AND Z = 0 CMP BX, CX B elow O R Equal S urely Above JBE BX_ BE

2 4.2.2 B X_BE ( BX i s B elow o r E qual) is a symbolic location JNBE / JA instruction if Not (Below or Equal) or if Above No Ex. Cy = 0 AND Z= 0 Cy = 1 OR Z = 1 CMP BX, CX Surely Above Below OR Equa l JBE BX_ BE J LE / J NG instruc tion if Less than OR Equal or J ump if Not Greater tha n No if [(S=1 AND V=0) OR (S=0 AND V=0) ] O R [(S=0 AND V=0) OR (S=1 AND V=1)] AND Z =1 Z = 0 [(surely negative) or (wrong answer positive! )] [(surel y positive) or (wrong answer negative!)] or Equal and not equal i. e. [ S XOR V=1] OR Z= 1 i. e. [S XOR V=0] AND Z= 0

3 J NLE / JG instruction if Not ( Less than OR Equal) or if Greater than J ump if No if [(S=0 AND V=0) OR (S=1 AND V=1)] AND [(S=1 AND V=0) OR ( S=0 AND V=1) ] O R Z =0 Z= 1 [(surely positive) or (wrong answer negative! )] [(surel y negative) or (wrong answer positive!)] and not equal o r equal e. S XOR V=0 AND Z= 0 e. S XOR V=1 OR Z= JL / JNGE instruction Ju mp L ess han t o r N OT Great ( er than o r E qual) J ump if N o J ump [ S = 1 A ND V =0] O R [ S = 0 A ND V = 1] [ S = 0 A ND V =0] O R [ S = 1 A ND V =1] ( surely n egative)or ( wrong a nswer ( surely p ositive) o r ( wrong a nswer p ositive! ) n egative! ) e. S X OR V=1 e N ote W hen S =1, result ca nnot b e 0. S X OR V= 0

4 4.2.5 J NL / JGE instruction if Not Less than or if Greater than OR E q ual No if [ S=0 AND V=0] OR (S=1 AND V = 1) [ S=1 AND V=0] OR (S=1 AND V = 1) ( surely positive) or (wrong answer negativ e!) ( surely negative) or (wrong answer positi v e! ) e. S XOR V= 0 e. S XOR V= 1 Note When S=0, result can be > = 0 U nconditional Ju mp i nstruction U nconditional Instruction Near Ju mp or Intra segment F ar o r Inter segment ( within the segment) ( to a different segme n t) N ear U nconditional Ju mp i nstruction N ear Jum p Direct (comm o n) Indirect (uncomm o n) 2- bytes Short J u mp (EB r8) 3- b ytes Long (E9 r16) 2 or more bytes

5 7 15 R ange + 2 R ange + 2 Starting with FFH R ange complete segment Three Near and two Far Ju mp instructions have the same m nemonic JMP, but they have different opcodes S hort J ump Instructio n 2 byte (EB r8) instructi on with Range to bytes For Backward jump Assembler knows the quantum of jump. Generates Short Jum p code if < =128 bytes is the required jump. Generates code for Long J u mp if >128 bytes is the required jump. For Forward ju mp Assembler doesn t know j ump quantum i n pass 1. Assembler reserve s 3 bytes for the forward jump instructio n. I f jump distance turns out to be >128 bytes, the instruction is coded as E9 r16 (E9H = Long j u mp code). If jump distance become s <=128 bytes, the instruction is coded as EB r8 followed by code for NOP (E8H = Short jump code).

6 4.2.5 SHORT Ass e mbler Directive Assembler generates only 2 byte Short code for forward jump, if the SHORT assembl e r directi ve is use d. JMP SHOR T SAME P rogrammer s hould e nsure t hat t he Ju mp d istance i s <=1 27 bytes S AME MOV CX, D X Long Jum p instruction 3-b yte (E9 r16) instruction with Range to byt e s L ong can cover entire 64K bytes of Code segment CS0000H CS8000H JMP FRWD L ong can handle it as jump quantum is <=32767 FRWD = CSFFFFH

7 L ong can handle it as jump B KWD= CS0000H quantum is <=32767 CS8000H JMP BKWD FRWD = CSFFFFH Lon g or Short? Can be treated as a CS0000H s mall ( 20H) Backward B ranch! CS0010H JMP FRWD J u mp distance =FFE0H. Too very long forward jump. FRWD = CS F FF0H CSFFFFH Can be treated as a smal l CS0000H ( 20H) F orward Branch! B KWD = CS0010H J u mp distance =FFE0H. T oo very long b ackward jump C SFFF0H JMP BKWD C SFFFFH

8 4.2.7 Intra s egment indirect Ju mp It is also called Near Indirect Ju mp. I t is not commonly used. I nstruction length 2 or more bytes Range complete segment E x.1 J MP DX If DX = 1234H, branches t o CS 1234H. 1234H is not signed relative displaceme nt. Ex. 2 JMP wordptr 2000 H[BX] I f BX contents is 1234H DS3234H 5678H Branches to CS5678H DS3236H AB22H F ar Ju mp ins tr uction F ar Direct (comm o n) Indirect (uncomm o n) 5 bytes, opcode EA, 2 byte offset, 2 or more bytes, 2 byte segme nt value starting wit h opcode FFH Range anywhere in memory Range anywhere in memor y As stated earlier, three Near and two Far instructions have the same mne m onic JMP but different opcodes.

9 4.2.9 Inter segment Direct instructi o n Also called Far Direct Ju mp. It I t is the common inter segment jump scheme is a 5 byte instructi on. 1 byte opcode (EAH), 2 byte offset value, 2 byte segment value Ex. J M P Far ptr LOC Inter segment Indirect instruct io n Also called Far Indirect. It is not commonly used. Instruction length depends on the w ay jump location is specifie d. It can be a minimum of 2 bytes. Ex. JM P DWORD PTR 2000H[BX] If BX contents is 1234H branch takes place to locatio n ABCDH 5678H. It is a 4-b yte instruction. DS3234H 5678H DS3236H ABCD H I teration Instructions I teration instructions provide a convenient way to impl ement loops in a program Iteration instruction s LOOP LOOPZ o r LOOPE LOOPNZ o r LOOPNE JC XZ

Lecture 1. How to Boot a PC. Andrei Pitiș. October 8, Operating Systems Practical. OSP Lecture 1, Booting 1/30

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