4 ms·

At least in Freescale's PowerPC documentation, it's convention to number the bits left-to-right in big-endian. So the most-significant bit is bit 0, which matc

At least in Freescale's PowerPC documentation, it's convention to number the bits left-to-right in big-endian. So the most-significant bit is bit 0, which matches up with the most-significant big-endian byte being 0. See, for a random example, page 1101 of https://www.nxp.com/docs/en/reference-manual/MPC8379ERM.pdf https://www.nxp.com/docs/en/reference-manual/MPC8379ERM.pdf. Personally I prefer the little-endian representation.

kazinator 9y ago
The documentation might have that numbering, but that makes no difference in programming on the PowerPC. When we take the value 1 and shift left by 1 bit, we get 2. That the documentation thinks this is bit 7 going to bit 6 is immaterial. Calling the MSB "bit 1" is a tip of the hat to serial communications. In serial communication and networking, it is predominant to transmit the MSB first. If the documentation is about a wire format, then using that numbering is correct down to the data link layer and (modulo framing considerations and such), physical.
- phs2501 9y ago
  Oh, I'm aware it makes no difference internally; I picked that documentation because I am somewhat intimately familiar with it as I worked on a product with that CPU and did a lot of driver work. My only point was to the poster I was replying to, who wrote "because when you number the bytes and bits you will see that the bytes are written left-to-right, while at the same time the bits are written right-to-left". That assumes that bits are universally numbered starting with 0 at the LSB, which isn't true.
ajdlinux 9y ago
MSB0 is convention in basically all IBM documentation (including PowerPC/Power Architecture stuff), hence all the Freescale/NXP PPC manuals follow. The main practical problem with MSB0 is that you need to consider the width of whatever field or register you're looking at to work out the correct bit shift.

kazinator 9y ago

The documentation might have that numbering, but that makes no difference in programming on the PowerPC. When we take the value 1 and shift left by 1 bit, we get 2. That the documentation thinks this is bit 7 going to bit 6 is immaterial. Calling the MSB "bit 1" is a tip of the hat to serial communications. In serial communication and networking, it is predominant to transmit the MSB first. If the documentation is about a wire format, then using that numbering is correct down to the data link layer and (modulo framing considerations and such), physical.

phs2501 9y ago
Oh, I'm aware it makes no difference internally; I picked that documentation because I am somewhat intimately familiar with it as I worked on a product with that CPU and did a lot of driver work. My only point was to the poster I was replying to, who wrote "because when you number the bytes and bits you will see that the bytes are written left-to-right, while at the same time the bits are written right-to-left". That assumes that bits are universally numbered starting with 0 at the LSB, which isn't true.

ajdlinux 9y ago

MSB0 is convention in basically all IBM documentation (including PowerPC/Power Architecture stuff), hence all the Freescale/NXP PPC manuals follow. The main practical problem with MSB0 is that you need to consider the width of whatever field or register you're looking at to work out the correct bit shift.