Trick: Reversing bits using hw

8

8bitforce

Member
If you need to reverse bits, FLEXIO module already has several registers that do it automatically. You write it to one register and can read it from a few others that have different bit flippings... This was very handy for my transputer project where I have to reverse 32bits constantly. FYI.

Code: 
// Use FLEXIO3 aliased buffers to flip bits for free :)
                                // (IMXRT_FLEXIO3_ADDRESS = 4202_0000h)
  FLEXIO3_SHIFTBUF0 = x;        // d[31:0] 200h + (a * 4h)
  x = FLEXIO3_SHIFTBUFBIS0;     // returns d[0:31] 280h + (a * 4h)
 
This might be handy if you wanted to reverse a value without involving the CPU (e.g. using DMA) but otherwise the rbit opcode is going to be much faster.
 
Thank you. I learned something new. I'm still stuck in 1980's.

Is there a better way to implement it than below? There is an __RBIT(uint32_t value) defined in cmsis_gcc.h.

Code: 
uint32_t value = 0x12345678;
uint32_t result;

asm volatile ("rbit %0, %1" : "=r" (result) : "r" (value) );

Serial.println(value, HEX);
Serial.println(result, HEX);
 
Unfortunately no.
You can remove "volatile" to give the compiler a bit more leeway for scheduling, but the best you can do to try to keep the code portable is wrapping it in a macro that throws a compile error for any machine that isn't ARM. People have long requested a GCC builtin for bit reversal but as of yet it's only appeared in clang.
 
You must log in or register to reply here.
Back
Top