{"id":99465,"date":"2018-08-10T07:00:00","date_gmt":"2018-08-10T21:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/?p=99465"},"modified":"2019-03-13T00:38:14","modified_gmt":"2019-03-13T07:38:14","slug":"20180810-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20180810-00\/?p=99465","title":{"rendered":"The PowerPC 600 series, part 5: Rotates and shifts"},"content":{"rendered":"

The Swiss army knife instruction of the PowerPC 600 series instruction set is rlwinm<\/code>, which stands for “rotate left word immediate and mask.” <\/p>\n

\n    rlwinm  rd, ra, imm5a, imm5b, imm5c\n    rlwinm. rd, ra, imm5a, imm5b, imm5c ; also updates cr0\n<\/pre>\n
This instruction does everything except wash your floor. <\/p>\n
First it takes the current value of the ra<\/var> register and rotates it left by imm5a<\/var> bits. Then it keeps all the bits imm5b<\/var> to imm5c<\/var>, inclusive, and clears all the other bits. If imm5b<\/var> is greater than imm5c<\/var>, the bits to keep wrap around! (Don’t forget that bit zero is the most significant bit.) The result of all this is placed in the rd<\/var> register. <\/p>\n
Let’s take a few examples. <\/p>\n
\n    rlwinm  rd, ra, 5, 6, 20\n<\/pre>\n
Here, we take ra<\/var>, rotate it left by 5 bits, and then keep bits 6 through 20. Since bit 0 is the most significant bit, then that means that the mask is <\/p>\n\n\n\n\n
0<\/td>\n1<\/td>\n2<\/td>\n3<\/td>\n4<\/td>\n5<\/td>\n6<\/td>\n7<\/td>\n8<\/td>\n9<\/td>\n10<\/td>\n11<\/td>\n12<\/td>\n13<\/td>\n14<\/td>\n15<\/td>\n16<\/td>\n17<\/td>\n18<\/td>\n19<\/td>\n20<\/td>\n21<\/td>\n22<\/td>\n23<\/td>\n24<\/td>\n25<\/td>\n26<\/td>\n27<\/td>\n28<\/td>\n29<\/td>\n30<\/td>\n31<\/td>\n<\/tr>\n
0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n<\/tr>\n
0<\/code><\/td>\n3<\/code><\/td>\nF<\/code><\/td>\nF<\/code><\/td>\nF<\/code><\/td>\n8<\/code><\/td>\n0<\/code><\/td>\n0<\/code><\/td>\n<\/tr>\n<\/table>\n
This comes out to 0x03FFF800<\/code>. Therefore, the result of the operation is <\/p>\n
\n     rd = rotl(ra, 5) & 0x03FFF800\n<\/pre>\n
The other case is where the bit count wraps around: <\/p>\n
\n    rlwinm  rd, ra, 5, 20, 6\n<\/pre>\n
This time, we start by setting bit 20, and continue to the end of the word, and then wrap around and start setting bits starting at bit 0, and then finally stop when we get to bit 6. <\/p>\n\n\n\n\n
0<\/td>\n1<\/td>\n2<\/td>\n3<\/td>\n4<\/td>\n5<\/td>\n6<\/td>\n7<\/td>\n8<\/td>\n9<\/td>\n10<\/td>\n11<\/td>\n12<\/td>\n13<\/td>\n14<\/td>\n15<\/td>\n16<\/td>\n17<\/td>\n18<\/td>\n19<\/td>\n20<\/td>\n21<\/td>\n22<\/td>\n23<\/td>\n24<\/td>\n25<\/td>\n26<\/td>\n27<\/td>\n28<\/td>\n29<\/td>\n30<\/td>\n31<\/td>\n<\/tr>\n
1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n0<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n1<\/td>\n<\/tr>\n
F<\/code><\/td>\nE<\/code><\/td>\n0<\/code><\/td>\n0<\/code><\/td>\n0<\/code><\/td>\nF<\/code><\/td>\nF<\/code><\/td>\nF<\/code><\/td>\n<\/tr>\n<\/table>\n
The net result is therefore <\/p>\n
\n     rd = rotl(ra, 5) & 0xFE000FFF\n<\/pre>\n
The Windows debugger for PowerPC was not in use for very long, and consequently its disassembler isn’t particularly advanced. But one thing it does do for you is unpack the last two parameters and tell you what the final mask is. <\/p>\n
\n10ae222c 54642d0c rlwinm  r4, r3,5,20,6            MASK=0xfe000fff\n<\/pre>\n
(Yes, the disassembler can’t make up its mind whether it puts a space after a comma or not. Like I said, the debugger didn’t last long enough to go through multiple iterations of polish.) <\/p>\n
With the rlwinmi<\/code> instruction, we can build many things. (In all examples, bit index arithmetic wraps around at 32.) <\/p>\n