The Hardware/Software Interface CSE351 Spring 2013 x86 Programming - PowerPoint PPT Presentation

University of Washington The Hardware/Software Interface CSE351 Spring 2013 x86 Programming II

University of Washington Today’s Topics: control flow  Condition codes  Conditional and unconditional branches  Loops 2

University of Washington Conditionals and Control Flow  A conditional branch is sufficient to implement most control flow constructs offered in higher level languages if (condition) then {...} else {…}  while (condition ) {…}  do {…} while (condition)  for (initialization; condition; iterative) {...}   Unconditional branches implement some related control flow constructs break, continue   In x86, we’ll refer to branches as “jumps” (either conditional or unconditional) 3

University of Washington Jumping  jX Instructions  Jump to different part of code depending on condition codes jX Condition Description jmp 1 Unconditional je ZF Equal / Zero jne ~ZF Not Equal / Not Zero js SF Negative jns ~SF Nonnegative jg ~(SF^OF)&~ZF Greater (Signed) jge ~(SF^OF) Greater or Equal (Signed) jl (SF^OF) Less (Signed) jle (SF^OF)|ZF Less or Equal (Signed) ja ~CF&~ZF Above (unsigned) jb CF Below (unsigned) 4

University of Washington Processor State (IA32, Partial) %eax  Information about currently executing %ecx program %edx General purpose  Temporary data %ebx registers ( %eax , …) %esi  Location of runtime %edi stack ( %ebp,%esp ) %esp Current stack top  Location of current %ebp Current stack frame code control point ( %eip ) %eip Instruction pointer  Status of recent tests ( CF,ZF,SF,OF ) CF ZF SF OF Condition codes 5

University of Washington Condition Codes (Implicit Setting)  Single-bit registers CF Carry Flag (for unsigned) SF Sign Flag (for signed) ZF Zero Flag OF Overflow Flag (for signed)  Implicitly set (think of it as side effect) by arithmetic operations Example: addl/addq Src,Dest ↔ t = a+b  CF set if carry out from most significant bit (unsigned overflow)  ZF set if t == 0  SF set if t < 0 (as signed)  OF set if two’s complement (signed) overflow (a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0)  Not set by lea instruction (beware!)  Full documentation (IA32): http://www.jegerlehner.ch/intel/IntelCodeTable.pdf 6

University of Washington Condition Codes (Explicit Setting: Compare)  Single-bit registers CF Carry Flag (for unsigned) SF Sign Flag (for signed) ZF Zero Flag OF Overflow Flag (for signed)  Explicit Setting by Compare Instruction cmpl/cmpq 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 (as signed)  OF set if two’s complement (signed) overflow (a>0 && b<0 && (a-b)<0) || (a<0 && b>0 && (a-b)>0) 7

University of Washington Condition Codes (Explicit Setting: Test)  Single-bit registers CF Carry Flag (for unsigned) SF Sign Flag (for signed) ZF Zero Flag OF Overflow Flag (for signed)  Explicit Setting by Test instruction testl / testq Src2 , Src1 testl b,a like computing a & b without setting destination  Sets condition codes based on value of Src1 & Src2  Useful to have one of the operands be a mask  ZF set if a&b == 0  SF set if a&b < 0  testl %eax, %eax  Sets SF and ZF, check if eax is +,0,- 8

University of Washington Reading Condition Codes  SetX Instructions  Set a single byte to 0 or 1 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) 9

University of Washington Reading Condition Codes (Cont.)  SetX Instructions: %eax %ah %al Set single byte to 0 or 1 based on combination of %ecx %ch %cl condition codes %edx %dh %dl  One of 8 addressable byte registers  Does not alter remaining 3 bytes %ebx %bh %bl  Typically use movzbl to finish job %esi int gt (int x, int y) %edi { %esp return x > y; } %ebp Body: y at 12(%ebp), x at 8(%ebp) movl 12(%ebp),%eax # eax = y What does each of cmpl %eax,8(%ebp) # Compare x : y setg %al # al = x > y these instructions do? movzbl %al,%eax # Zero rest of %eax 10

University of Washington Reading Condition Codes (Cont.)  SetX Instructions: %eax %ah %al Set single byte to 0 or 1 based on combination of %ecx %ch %cl condition codes %edx %dh %dl  One of 8 addressable byte registers  Does not alter remaining 3 bytes %ebx %bh %bl  Typically use movzbl to finish job %esi int gt (int x, int y) %edi { %esp return x > y; } %ebp Body: y at 12(%ebp), x at 8(%ebp) movl 12(%ebp),%eax # eax = y cmpl %eax,8(%ebp) # Compare x and y (x – y) setg %al # al = x > y movzbl %al,%eax # Zero rest of %eax 11

University of Washington Jumping  jX Instructions  Jump to different part of code depending on condition codes jX Condition Description jmp 1 Unconditional je ZF Equal / Zero jne ~ZF Not Equal / Not Zero js SF Negative jns ~SF Nonnegative jg ~(SF^OF)&~ZF Greater (Signed) jge ~(SF^OF) Greater or Equal (Signed) jl (SF^OF) Less (Signed) jle (SF^OF)|ZF Less or Equal (Signed) ja ~CF&~ZF Above (unsigned) jb CF Below (unsigned) 12

University of Washington Conditional Branch Example absdiff: int absdiff(int x, int y) pushl %ebp { Setup movl %esp, %ebp int result; movl 8(%ebp), %edx if (x > y) { movl 12(%ebp), %eax result = x-y; cmpl %eax, %edx } else { Body1 jle .L7 result = y-x; subl %eax, %edx } movl %edx, %eax return result; .L8: } leave Finish ret .L7: subl %edx, %eax Body2 jmp .L8 13

University of Washington Conditional Branch Example (Cont.) int goto_ad(int x, int y) int absdiff(int x, int y) { { int result; int result; if (x <= y) goto Else ; if (x > y) { result = x-y; result = x-y; Exit : } else { return result; result = y-x; Else: } result = y-x; return result; goto Exit ; } } C allows “ goto ” as means of  transferring control  Closer to machine-level programming style Generally considered bad coding  style 14

University of Washington Conditional Branch Example (Cont.) absdiff: int goto_ad(int x, int y) pushl %ebp { movl %esp, %ebp int result; movl 8(%ebp), %edx if (x <= y) goto Else ; movl 12(%ebp), %eax result = x-y; cmpl %eax, %edx Exit : jle .L7 return result; subl %eax, %edx Else: movl %edx, %eax result = y-x; .L8: goto Exit ; leave } ret .L7: subl %edx, %eax int x %edx jmp .L8 int y %eax 15

University of Washington Conditional Branch Example (Cont.) absdiff: int goto_ad(int x, int y) pushl %ebp { movl %esp, %ebp int result; movl 8(%ebp), %edx if (x <= y) goto Else ; movl 12(%ebp), %eax result = x-y; cmpl %eax, %edx Exit : jle .L7 return result; subl %eax, %edx Else: movl %edx, %eax result = y-x; .L8: goto Exit ; leave } ret .L7: subl %edx, %eax int x %edx jmp .L8 int y %eax 16

University of Washington Conditional Branch Example (Cont.) absdiff: int goto_ad(int x, int y) pushl %ebp { movl %esp, %ebp int result; movl 8(%ebp), %edx if (x <= y) goto Else ; movl 12(%ebp), %eax result = x-y; cmpl %eax, %edx Exit : jle .L7 return result; subl %eax, %edx Else: movl %edx, %eax result = y-x; .L8: goto Exit ; leave } ret .L7: subl %edx, %eax int x %edx jmp .L8 int y %eax 17

University of Washington Conditional Branch Example (Cont.) absdiff: int goto_ad(int x, int y) pushl %ebp { movl %esp, %ebp int result; movl 8(%ebp), %edx if (x <= y) goto Else ; movl 12(%ebp), %eax result = x-y; cmpl %eax, %edx Exit : jle .L7 return result; subl %eax, %edx Else: movl %edx, %eax result = y-x; .L8: goto Exit ; leave } ret .L7: subl %edx, %eax int x %edx jmp .L8 int y %eax 18

University of Washington Conditional Branch Example (Cont.) absdiff: int goto_ad(int x, int y) pushl %ebp { movl %esp, %ebp int result; movl 8(%ebp), %edx if (x <= y) goto Else ; movl 12(%ebp), %eax result = x-y; cmpl %eax, %edx Exit : jle .L7 return result; subl %eax, %edx Else: movl %edx, %eax result = y-x; .L8: goto Exit ; leave } ret .L7: subl %edx, %eax int x %edx jmp .L8 int y %eax 19

University of Washington General Conditional Expression Translation if ( Test ) C Code val = Then-Expr ; val = Test ? Then-Expr : Else-Expr ; else val = Else-Expr ; result = x>y ? x-y : y-x;  Test is expression returning integer Goto Version = 0 interpreted as false  0 interpreted as true nt = !Test ; if (nt) goto Else ;  Create separate code regions for val = Then-Expr ; then & else expressions Done:  Execute appropriate one . . . Else: val = Else-Expr ;  How might you make this more goto Done; efficient? 20