CMU CS 15213 - Lecture (33 pages)

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Lecture



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Lecture

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Pages:
33
School:
Carnegie Mellon University
Course:
Cs 15213 - Introduction to Computer Systems
Introduction to Computer Systems Documents

Unformatted text preview:

15 213 The course that gives CMU its Zip Machine Level Programming II Control Flow Sept 14 2000 Topics Condition Codes Setting Testing Control Flow If then else Varieties of Loops Switch Statements class06 ppt Condition Codes Single Bit Registers CF ZF SF OF Carry Flag Zero Flag Sign Flag Overflow Flag Implicit Setting By Arithmetic Operations addl Src Dest C analog t a b CF set if carry out from most significant bit Used to detect unsigned overflow ZF set if t 0 SF set if t 0 OF set if two s complement overflow a 0 b 0 t 0 a 0 b 0 t 0 Not Set by leal instruction class06 ppt 2 CS 213 F 00 Setting Condition Codes cont Explicit Setting by Compare Instruction cmpl Src2 Src1 cmpl b a like computing a b without setting destination CF set if carry out from most significant bit Used for unsigned comparisons ZF set if a b SF set if a b 0 OF set if two s complement overflow a 0 b 0 a b 0 a 0 b 0 a b 0 Explicit Setting by Test instruction testl Src2 Src1 Sets condition codes based on value of Src1 Src2 Useful to have one of the operands be a mask testl b a like computing a b without setting destination ZF set when a b 0 SF set when a b 0 class06 ppt 3 CS 213 F 00 Reading Condition Codes SetX Instructions Set single byte based on combinations of condition codes SetX Condition Description sete ZF Equal Zero setne ZF Not Equal Not Zero sets SF Negative setns SF Nonnegative setg SF OF ZF Greater Signed setge SF OF Greater or Equal Signed setl SF OF Less Signed setle SF OF ZF Less or Equal Signed seta CF ZF Above unsigned setb CF Below unsigned class06 ppt 4 CS 213 F 00 Reading Condition Codes Cont SetX Instructions Set single byte based on combinations of condition codes One of 8 addressable byte registers Embedded within first 4 integer registers Does not alter remaining 3 bytes Typically use andl 0xFF eax to finish job eax ah al edx dh dl ecx ch cl ebx bh bl esi int gt int x int y return x y movl cmpl setg andl 12 ebp eax eax 8 ebp al 255 eax class06 ppt edi esp Body eax y



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