Honored ContributorI'm new to programming the Nios, and I'm stuck on what should be a simple task.
Is there a way in C, or with inline assembly instructions, to read/write to the 32 general purpose registers? I've found rdctl/wrctl and rdprs/wrprs. But how, for instance, do I store the address pointed to by the stack pointer into a variable? Thanks.
Honored ContributorYou don't!
The compiler uses registers for your variables ...Honored ContributorHonored ContributorAh ....
In that case you probably need to use 'stw' to save sp in memory, switch registers, then 'ldw' the value back - probably via offset from gp (which you want the same in both register sets). I don't think there is anywhere (other than memory) the sp value can be saved across the register switch - although I guess a custom instruction could save it somewhere. You have bigger problems if you are writing in C though - as gcc will have all sorts of values in registers! On he other hand, if you are worried enough about perfromance somewhere to be avoiding register save/restore, I'd have thought you'd want to avoid all stack accesses! I've been compiling code in a single source module so that gcc inlines all static function (anything called only once is always inlined), and taken care elsewhere that no locals end up on the stack - so the only stack accesses are saving the callee saved registers on entry (which never get restored because the function doesn't return). After specifying -fcall-used-rxx for r16-25,29-31, it is a shame you can't include sp itself! (nor fp or r1).
