1. COMP 2003: Assembly Language and Digital Logic Chapter 2: Flags and Instructions Notes by Neil Dickson

2. More Common Instructions sub dest,src;dest -= src and dest,src;dest &= src or dest,src;dest |= src xor dest,src;dest ^= src

3. Examples of and/or/xor mov al,0E3h and al,59h or al,7Ah xor al,26h xor al,al 11100011 al 59h 01010101 01000001 al 7Ah 01110110 01110111 al 26h 00101010 01011101 al 3 bits cleared 4 bits set 3 bits complemented 01011101 al 00000000 all set bits complemented, clearing them, so result is 0

4. Bit Shift Instructions shl dest,imm;dest <<= imm shr dest,imm;(unsigned)dest >>= imm shr dest,cl;(unsigned)dest >>= cl shl dest,cl;dest <<= cl sar dest,imm;(signed)dest >>= imm sar dest,cl;(signed)dest >>= cl 11100011000 Bit 0Bit 1Bit 2Bit 3Bit 4Bit 5Bit 6Bit 7 Shifting left 3 bits ;dest *= 2 imm ;dest *= 2 cl ;(unsigned)dest /= 2 imm ;(unsigned)dest /= 2 cl ;(signed)dest /= 2 imm ;(signed)dest /= 2 cl

5. mul & div (unsigned) mul src8bit;ax = al*src8bit mul src16bit;dx:ax = ax*src16bit mul src32bit;edx:eax = eax*src32bit div src8bit;al = ax/src8bit div src16bit;ax = dx:ax/src16bit div src32bit;eax = edx:eax/src32bit Divide error if result doesn’t fit

6. not, neg, & xchg not src;src = ~src neg src;src = -src xchg dest,src;swap(dest,src)

7. Main Condition Flags Zero Flag: 1 if result is zero, else 0 Carry Flag: 1 if operation had unsigned overflow (carry/borrow), else 0 Sign Flag: 1 if result is negative, else 0 Overflow Flag: 1 if operation had signed overflow, else 0 Parity Flag: 1 if low byte of result has even number of bits set, else 0

8. Compare / Test Instructions cmp dest,src;flags of dest-src test dest,src;flags of dest&src NOTE: add, sub, and, or, xor, shl, shr, sar, mul, div, not, and neg also modify the flags based on their results, but cmp and test only modify the flags.

9. cmp Conditions Equal = Zero (ZF=1) Not Equal = Not Zero (ZF=0) Below = Carry (CF=1) Not Below = Above or Equal = No Carry (CF=0) Above = No Carry and Not Zero (CF=0 and ZF=0) Not Above = Below or Equal = Carry or Zero (CF=1 or ZF=1) Less (signed) = (it’s complicated) Not Less (signed) = Greater or Equal (signed) Greater (signed) Not Greater (signed) = Less or Equal (signed) je, jz jne, jnz jb, jc jnb, jnc, jae ja jna, jbe jl jnl, jge jg jng, jle sete, setz, cmove, cmovz setne, setnz, cmovne,...

10. Other Conditions negative Sign (SF=1) No negative Sign (SF=0) Overflow (OF=1) No Overflow (OF=0) Parity = Parity Even (PF=1) Not Parity = Parity Odd (PF=0) js jns jo jp, jpe sets, cmovs setns, cmovns jno jnp, jpo Remember: cmp = sub that only saves flags (ZF, CF, SF, OF) for comparing values test = and that only saves flags (ZF, SF; CF=0, OF=0) for checking whether certain bits are set...

11. Example: Counting Set Bits count = 0; for (int i=0;i<32;++i) { // for each bit i if (data & (1<<i)) { // if bit i is 1 ++count; // add 1 to count } edx = 0; cl = 0; do { if (eax & (1<<cl)) { ++edx; } ++cl; } while (cl<32); edx = 0; cl = 0; NextBit: ebx = 1; ebx <<= cl; if ((eax & ebx)==0) goto BitIsZero; ++edx; BitIsZero: ++cl; if (cl<32) goto NextBit; ↓ →

12. Example: Counting Set Bits edx = 0; cl = 0; NextBit: ebx = 1; ebx <<= cl; if ((eax & ebx)==0) goto BitIsZero; ++edx; BitIsZero: ++cl; if (cl<32) goto NextBit; mov edx,0 mov cl,0 NextBit: mov ebx,1 shl ebx,cl test eax,ebx jz BitIsZero add edx,1 BitIsZero: add cl,1 cmp cl,32 jb NextBit → 1 1 After this test, the zero flag is set if the result of ( eax & ebx ) is 0, else zero flag is clear. 2 2 cmp dest,src followed by jb LineLabel means that the code jumps iff dest =src) We can improve upon this!

13. Starting to Improve mov edx,0 mov cl,0 NextBit: mov ebx,1 shl ebx,cl test eax,ebx jz BitIsZero add edx,1 BitIsZero: add cl,1 cmp cl,32 jb NextBit 1 1 This bit mask in ebx gets recalculated from scratch every time, when each time the bit that’s 1 is just one bit to the left from last time. mov edx,0 mov cl,0 mov ebx,1 NextBit: test eax,ebx jz BitIsZero add edx,1 BitIsZero: shl ebx,1 add cl,1 cmp cl,32 jb NextBit 2 2 cl is now only used to count the number of times that the loop occurs, so count down from 32 to 0 instead of up from 0 to 32 so that we can just check the zero flag to see if it’s zero yet. mov edx,0 mov cl,32 mov ebx,1 NextBit: test eax,ebx jz BitIsZero add edx,1 BitIsZero: shl ebx,1 sub cl,1 jnz NextBit Jumps if cl (the result of sub ) isn’t zero yet.

14. Another Approach mov edx,0 ;count = 0 NextBit: ;do { test eax,1 ; if (bit 0 of eax is set) { jz BitIsZero ; add edx,1 ; ++count BitIsZero: ; } shr eax,1 ; shift eax right to get a new bit 0 jnz NextBit ;} while (eax!=0) 1 1 The value of eax gets destroyed, so this may not be desired, but we could always mov it into another register beforehand. 2 2 This is the same as checking whether eax is odd, since in binary, odd numbers have a 1 in bit 0, and even numbers have a 0 in bit 0. 3 3 As mentioned before, this is the same as dividing eax by 2. Went from 8 instructions in the loop down to 5, and <32 iterations if high bit of eax is clear