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Hello, As far as the V flag is concerned, I've taken a look at the Sparc V8 reference manual (www.sparc.org/standards/V8.pdf) We can read at page 170 for the update of the V flag for "addcc" and "addxcc": Vtheory = (r[rs1]<31> & operand2<31> & !result<31>) | (!r[rs1]<31> & !operand2<31> && result<31>) Let's transform this with the name of the variables in the qemu code : Vtheory = (src1<31> & T1<31> & !T0<31>) | (!src1<31> & !T1<31> & T0<31>) Vtheory = ((src1 & T1 & ~T0) | (~src1 & ~T1 & T0)<31> And we have in qemu code : Vqemu = ((src1 ^ T1 ^ -1) & (src1 ^ T0))<31> Now, let's transform Vqemu : Vqemu = ((src1 ^ (T1 ^ -1)) & (src1 ^ T0))<31> Vqemu = ((src1 ^ ~T1) & (src1 ^ T0))<31> Vqemu = (((src1 & ~(~T1)) | (~src1 & ~T1)) & (src1 ^ T0))<31> Vqemu = (((src1 & T1) | (~src1 & ~T1)) & (src1 ^ T0))<31> Vqemu = ((src1 & T1 & (src1 ^ T0)) | (~src1 & ~T1 & (src1 ^ T0)))<31> Vqemu = ((src1 & T1 & ((src1 & ~T0) | (~src1 & T0))) | (~src1 & ~T1 & ((src1 & ~T0) | (~src1 & T0))))<31> Vqemu = ((src1 & T1 & src1 & ~T0) | (src1 & T1 & ~src1 & T0) | (~src1 & ~T1 & src1 & ~T0) | (~src1 & ~T1 & ~src1 & T0))<31> Vqemu = ((src1 & T1 & ~T0) | (~src1 & ~T1 & T0))<31> Vqemu = Vtheroy ! After theory, a bit of practice! I just wrote a small piece of code that enumerates the 2*2*2=8 combinations and proves experimentally that Vqemu = Vtheroy. int main(int argc, char* argv[]) { int src1, T1, T0; for(src1=0;src1<=1;src1++) { for(T1=0;T1<=1;T1++) { for(T0=0;T0<=1;T0++) { int V1 = (src1 & T1 & ~T0) | (~src1 & ~T1 & T0); int V2 = (src1 ^ T1 ^ 1) & (src1 ^ T0); printf("src1=%d T1=%d T0=%d, V=%d=%d\n", src1, T1, T0, V1, V2); } } } } The output is : src1=0 T1=0 T0=0, V=0=0 src1=0 T1=0 T0=1, V=1=1 src1=0 T1=1 T0=0, V=0=0 src1=0 T1=1 T0=1, V=0=0 src1=1 T1=0 T0=0, V=0=0 src1=1 T1=0 T0=1, V=0=0 src1=1 T1=1 T0=0, V=1=1 src1=1 T1=1 T0=1, V=0=0 In other words, the V flag is set when :
Conclusion : the computation of the V flag in qemu is correct, and their is no special case to consider if the C flag is set or not :-) For tomorrow, the formal proof of the correctness of the whole qemu code ;-) Le Jeudi 13 Avril 2006 20:39, vous avez écrit : > >As far as the V flag is concerned, mmm, I'm not really sure whether we > >should > >change something in the sparc code. If we compare to the arm code, we > > don't take into account the fact that the carry flag is set before. > > > >We'd probably need some extensive tests and their associated expected > >results. > > I made a small test program (attached) to test the addx instruction. The > program calculates the sum of two 64-bit values, given on the command line > as 32-bit lower and upper parts. Native system produces following: > $ ./addx -1 -1 0x80000000 -1 > ffffffffffffffff + ffffffff80000000 = ffffffff7fffffff, NZVC: 9 > while unpatched Qemu the following: > $ qemu-sparc ./addx -1 -1 0x80000000 -1 > ffffffffffffffff + ffffffff80000000 = ffffffff7fffffff, NZVC: 8 > > So the carry flag not set. When your patch is applied, the output is > identical: > ffffffffffffffff + ffffffff80000000 = ffffffff7fffffff, NZVC: 9 > > I couldn't think of a combination of values that would set the V flag when > there is also a carry from the 32-bit addition, any suggestions? > > _________________________________________________________________ > FREE pop-up blocking with the new MSN Toolbar - get it now! > http://toolbar.msn.click-url.com/go/onm00200415ave/direct/01/ |
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