Bit band calculation for cortex M3/4

[ivand58]

The benefit of Bit-banding is that a write to a word in the alias region performs a write to the corresponding bit in the Bit-band region. Also, reading a word in the alias region will return the value of the corresponding bit in the Bit-band region. These operations take a single machine instruction thus eliminate race conditions. This is especially useful for interacting with peripheral registers where it is often necessary to set and clear individual bits.

[MitchBradley]

bitband.fth is specific to a particular processor family so it should not be in the generic src/cforth/lib/directory. A better place would be src/cpu/arm/cortex-m3

[MitchBradley]

: bitband  ( bit adr -- aliasadr )
   dup 5 lshift                      ( bit adr byte-offset )   \ Spread low part of address; high bits will be shifted out
   swap $f000.0000 and or  ( bit adr' )                     \ Keep high nibble of address
   $0200.0000 or                  ( bit adr' )                    \ Offset to bitband alias space
   swap la+                          ( aliasadr )                    \ Merge bit number as 32-bit word offset
;

The above is smaller, faster and more direct. There is no value in declaring the masks as constants because they are fixed for this architecture. The value of having symbolic names is nullified by the confusion caused by the similarity of the names that differ only in a trailing '_', which has no mnemonic value.

But, all that said, if one is to use the bitbanding feature, why not just define the bit object address within the bitband region directly, avoiding the address,bit# representation entirely?

$4222.0234 constant mybit
1 mybit l!

[MitchBradley]

Fixing the formatting:

: bitband  ( bit adr -- aliasadr )
   dup 5 lshift            ( bit adr byte-offset )  \ Spread low part of address; high bits will be shifted out
   swap $f000.0000 and or  ( bit adr' )             \ Keep high nibble of address
   $0200.0000 or           ( bit adr' )             \ Offset to bitband alias space
   swap la+                ( aliasadr )             \ Merge bit number as 32-bit word offset
;

[ivand58]

what about the following usecase:

VARIABLE x
\ ...
1 x BITBAND
1 SWAP C!

In this case it is difficult to type the address of the bit that corresponds to the address of the variable.

[quozl]

For interest, my Teensy 3.x GPIO file at https://github.com/MitchBradley/cforth/blob/master/src/platform/arm-teensy3/gpio.fth is a bit band implementation but without mentioning bit band. 😁