Introduction to the StrongARM Revision 3, 04-Oct-96
The StrongARM 110 is a new high-performance processor from Digital Semiconductor. It offers performance improvements over the ARM710 ranging from 100% to 1000%, depending on the application.
A StrongARM processor card will available for the Risc PC later this year. It will come with a new ROM set containing RISC OS 3.70, a new StrongARM- compatible version of RISC OS. These documents provide information on writing software, and modifying existing software, for StrongARM-based machines.
Significant StrongARM features
- 202MHz core clock
- 5-stage pipeline (Fetch, Issue, Execute, Buffer, Write)
- Separate 16K write-back data cache and 16K instruction cache
- 8 entry write buffer, each entry holding 1-16 bytes
- Fast 32 and 64 bit result multiply instructions
- More instructions per cycle than previous ARMs
Incompatibilities
The StrongARM has two significant differences from previous ARMs that can affect existing programs. Firstly, the split caches mean that instructions written into memory may linger in the data cache, and not be in real memory when the instruction cache comes to fetch them. Likewise if some code is already in the instruction cache, and it is altered, the changed code will not be noticed. Thus existing self-modifying code and dynamic code creation or loading will generally not work.
RISC OS 3.7 provides a call to synchronise the instruction and data caches, but this is a slow operation on the StrongARM. Self-modifying code should therefore be avoided. The most common failures here arise from custom SWI veneers that assemble code on the stack, dynamic code loading and linking and custom code squeezing and encryption.
The other significant StrongARM change is that storing the PC to memory (using STR or STM) stores PC+8 rather than PC+12. This can generally be catered for by judicious use of NOPs to allow for both possibilities. This affects, for example, APCS stack backtraces and vector claimants (the example code on PRM page 1-107 will not work, for example).
There are other differences that are only likely to be encountered by low-level system code.
Most applications will run unmodified on StrongARM, especially those written in C and BASIC, but some code will inevitably need modification.
What to read next
Guidelines gives a general overview of guidelines for authoring software for use on StrongARM based machines.
SAsupport details the new and extended SWIs in RISC OS 3.7 to support the StrongARM.
MiscChange details other changes in RISC OS 3.7 that are not related to StrongARM compatibility.
Performance gives hints on how to improve performance of code on StrongARM.
FIQs shows how to get FIQ code working on StrongARM.