Sep 082012

Make your 32-bit computer do an Impression (ho-ho) of a 26-bit one!

When Castle Technology Ltd launched the IYONIX pc, back in 2002, there was a significant question users needed an answer to before upgrading to the new computer: Would their old software run on the new hardware?

The problem was that for all the previous RISC OS computers, the ARM CPUs worked in (or supported in the case of StrongARM) an addressing mode we refer to as ’26-bit’, in which the program counter and processor status flags are contained in a single register; six bits are used for the status flags, and 26 bits for the program counter – the pointer to where in memory instructions are read for execution. With instructions always being word-aligned, rather than byte-aligned, the 26-bit program counter actually provides a 28-bit address range, representing bits 2 to 27 in the actual address – the lower two bits pointing to the instruction to be read are always zero. Continue reading »