From: "Eric Lemoine" <redacted>
Date: Tue, 14 Nov 2006 08:28:42 +0100

because it makes it explicit that only bits 0 through 6 are taken into
account when writing the IACK register.

The phrase "bits 0 through 6" doesn't make any sense when bit 3 DOES
NOT EXIST in the hardware, it's reserved, it's not there, so including
it only confuses people and obfuscates the code.

Please use the explicit bit mask composed of existing macros, which
not only makes sure that the mask has meaning, but it also makes sure
that reserved and non-existing bits are never referenced.