Masking

What is a 'mask' ? In programming, a mask determines which bits we want to keep and which bits we want to clear. To do this, the bits we want to keep are anded with the data we are interested in.

In this case a mask is needs to be created from the BitWidth.

Generate a bitfield mask based on a bitWidth:
: bwm ( bitWidth -- mask ) bit 1 - ;

Example:
2 bfm bin.
3322222222221111111111
10987654321098765432109876543210
00000000000000000000000000000011

see bwm
0000EC0C: 2301  movs r3 #1
0000EC0E: 40B3  lsls r3 r6
0000EC10: 3B01  subs r3 #1
0000EC12: 461E  mov r6 r3
0000EC14: 4770  bx lr
Bytes: 10  ok.