Instructor: - MACHINE-LEVEL PROGRAMMING II: ARITHMETIC & CONTROL2 University of Texas at Austin Today • Complete addressing mode, address computation (leal) • Arithmetic operations • Control: Condition codes • Conditional branches • While loops3 University of Texas at Austin Complete Memory Addressing Modes • Most General Form • D(Rb,Ri,S) Mem[Reg[Rb]+S*Reg[Ri]+ D] • D: Constant “displacement” 1, 2, or 4 bytes • Rb: Base register: Any of 8 integer registers • Ri: Index register: Any, except for %esp • Unlikely you’d use %ebp, either • S: Scale: 1, 2, 4, or 8 (why these numbers?) • Special Cases • (Rb,Ri) Mem[Reg[Rb]+Reg[Ri]] • D(Rb,Ri) Mem[Reg[Rb]+Reg[Ri]+D] • (Rb,Ri,S) Mem[Reg[Rb]+S*Reg[Ri]]4 University of Texas at Austin Address Computation Examples Expression Address Computation Address 0x8(%edx) 0xf000 + 0x8 0xf008 (%edx,%ecx) 0xf000 + 0x100 0xf100 (%edx,%ecx,4) 0xf000 + 4*0x100 0xf400 0x80(,%edx,2) 2*0xf000 + 0x80 0x1e080 %edx 0xf000 %ecx 0x0100 Expression Address Computation Address 0x8(%edx) (%edx,%ecx) (%edx,%ecx,4) 0x80(,%edx,2)5 University of Texas at Austin Address Computation Instruction • leal Src,Dest • Src is address mode expression • Set Dest to address denoted by expression • Uses • Computing addresses without a memory reference • E.g., translation of p = &x[i]; • Computing arithmetic expressions of the form x + k*y • k = 1, 2, 4, or 8 • Example int mul12(int x) { return x*12; } leal (%eax,%eax,2), %eax ;t <- x+x*2 sall $2, %eax ;return t<<2 Converted to ASM by compiler:6 University of Texas at Austin Today • Complete addressing mode, address computation (leal) • Arithmetic operations • Control: Condition codes • Conditional branches • While loops7 University of Texas at Austin Some Arithmetic Operations • Two Operand Instructions: Format Computation addl Src,Dest Dest = Dest + Src subl Src,Dest Dest = Dest  Src imull Src,Dest Dest = Dest * Src sall Src,Dest Dest = Dest << Src Also called shll sarl Src,Dest Dest = Dest >> Src Arithmetic shrl Src,Dest Dest = Dest >> Src Logical xorl Src,Dest Dest = Dest ^ Src andl Src,Dest Dest = Dest & Src orl Src,Dest Dest = Dest | Src • Watch out for argument order! • No distinction between signed and unsigned int (why?)8 University of Texas at Austin Some Arithmetic Operations • One Operand Instructions incl Dest Dest = Dest + 1 decl Dest Dest = Dest  1 negl Dest Dest =  Dest notl Dest Dest = ~Dest • See book for more instructions9 University of Texas at Austin Arithmetic Expression Example int arith(int x, int y, int z) { int t1 = x+y; int t2 = z+t1; int t3 = x+4; int t4 = y * 48; int t5 = t3 + t4; int rval = t2 * t5; return rval; } arith: pushl %ebp movl %esp, %ebp movl 8(%ebp), %ecx movl 12(%ebp), %edx leal (%edx,%edx,2), %eax sall $4, %eax leal 4(%ecx,%eax), %eax addl %ecx, %edx addl 16(%ebp), %edx imull %edx, %eax popl %ebp ret Body Set Up Finish10 University of Texas at Austin • • • 16 z 12 y 8 x 4 Rtn Addr 0 Old %ebp Understanding arith movl 8(%ebp), %ecx movl 12(%ebp), %edx leal (%edx,%edx,2), %eax sall $4, %eax leal 4(%ecx,%eax), %eax addl %ecx, %edx addl 16(%ebp), %edx imull %edx, %eax %ebp Offset int arith(int x, int y, int z) { int t1 = x+y; int t2 = z+t1; int t3 = x+4; int t4 = y * 48; int t5 = t3 + t4; int rval = t2 * t5; return rval; }11 University of Texas at Austin • • • 16 z 12 y 8 x 4 Rtn Addr 0 Old %ebp Understanding arith %ebp Offset Stack int arith(int x, int y, int z) { int t1 = x+y; int t2 = z+t1; int t3 = x+4; int t4 = y * 48; int t5 = t3 + t4; int rval = t2 * t5; return rval; } movl 8(%ebp), %ecx # ecx = x movl 12(%ebp), %edx # edx = y leal (%edx,%edx,2), %eax # eax = y*3 sall $4, %eax # eax *= 16 (t4) leal 4(%ecx,%eax), %eax # eax = t4 +x+4 (t5) addl %ecx, %edx # edx = x+y (t1) addl 16(%ebp), %edx # edx += z (t2) imull %edx, %eax # eax = t2 * t5 (rval)12 University of Texas at Austin Observations about arith • Instructions in different order from C code • Some expressions require multiple instructions • Some instructions cover multiple expressions • Get exact same code when compile: • (x+y+z)*(x+4+48*y) movl 8(%ebp), %ecx # ecx = x movl 12(%ebp), %edx # edx = y leal (%edx,%edx,2), %eax # eax = y*3 sall $4, %eax # eax *= 16 (t4) leal 4(%ecx,%eax), %eax # eax = t4 +x+4 (t5) addl %ecx, %edx # edx = x+y (t1) addl 16(%ebp), %edx # edx += z (t2) imull %edx, %eax # eax = t2 * t5 (rval) int arith(int x, int y, int z) { int t1 = x+y; int t2 = z+t1; int t3 = x+4; int t4 = y * 48; int t5 = t3 + t4; int rval = t2 * t5; return rval; }13 University of Texas at Austin Another Example int logical(int x, int y) { int t1 = x^y; int t2 = t1 >> 17; int mask = (1<<13) - 7; int rval = t2 & mask; return rval; } logical: pushl %ebp movl %esp,%ebp movl 12(%ebp),%eax xorl 8(%ebp),%eax sarl $17,%eax andl $8185,%eax popl %ebp ret Body Set Up Finish movl 12(%ebp),%eax # eax = y xorl 8(%ebp),%eax # eax = x^y (t1) sarl $17,%eax # eax = t1>>17 (t2) andl $8185,%eax # eax = t2 & mask (rval)14 University of Texas at Austin Another Example int logical(int x, int y) { int t1 = x^y; int t2 = t1 >> 17; int mask = (1<<13) - 7; int rval = t2 & mask; return rval; } logical: pushl %ebp movl %esp,%ebp movl 12(%ebp),%eax xorl 8(%ebp),%eax sarl $17,%eax andl $8185,%eax popl %ebp ret Body Set Up Finish movl 12(%ebp),%eax # eax = y xorl 8(%ebp),%eax # eax = x^y (t1) sarl $17,%eax # eax = t1>>17 (t2) andl $8185,%eax # eax = t2 & mask (rval)15 University of Texas at Austin Another Example int logical(int x, int y) { int t1 = x^y; int t2 = t1 >> 17; int mask = (1<<13) - 7; int rval = t2 & mask; return rval; } logical: pushl %ebp movl %esp,%ebp movl 12(%ebp),%eax xorl 8(%ebp),%eax sarl $17,%eax andl $8185,%eax popl %ebp ret Body Set Up Finish movl 12(%ebp),%eax # eax = y xorl 8(%ebp),%eax # eax = x^y (t1) sarl $17,%eax # eax = t1>>17 (t2) andl $8185,%eax # eax = t2 & mask (rval)16 University of Texas at Austin Another Example int logical(int x, int y) { int t1 = x^y; int t2 = t1 >> 17; int mask = (1<<13) - 7; int rval = t2 & mask; return rval; } logical: pushl