New global header file available

One for assembler programmers, this…

RISC OS programs are generally expected to run on the widest possible range of ARM chips, right from the ARM2 (ARM architecture v2) up to the Cortex-A8 (ARM architecture v7). In an attempt to support the full range of CPUs, software is often forced to forego new CPU features, or else invoke conditional or branched execution at run-time to cater for all CPUs. Either way, it means you’re unlikely to get the best performance possible.

There is another way, though: build-time switches can target the binary to one or more specific CPUs. Whilst the performance gain is not worth the administative overhead of maintaining multiple binary variants for most disc components (with the notable exception of CPU-intensive things like codecs and graphics renderers), there is no good reason why ROM builds shouldn’t be more closely targetted at the relevant CPU than they have traditionally been.

The best technique the build system has provided for this so far is the Machine build variable, which you set by choosing an Env file (or by selecting an environment from the menu in !Builder). But relying on this means that references to increasingly obscure codenames get scattered through the source tree, and these need revisiting as each new platform is added – and even more care has to be taken if each new target isn’t a strict superset of the previous one, as has happened several times in the past.

The new header file, Hdr:CPU.Arch provides a central location where each Machine setting is mapped to a particular range of architecture features which it is expected that binaries produced by that build must support. By including this header file, you can write code which is either highly targetted to one CPU, or is broadly applicable, and choose which version to build into your binary based upon clearly defined and well-documented build switches, rather than a list of hardware platform codenames.

You can examine the header file here. Examples of its use are included in comments.