Struct interface for IO doesn't work

[Altera_Forum]

I was trying to convert a large code base written for the NIOS I processor to NIOS II. My clever idea was to simply include the excalibur.h and compile for NIOS II. However, every time I read from a memory address through the old struct interface of NIOS I, the compiler uses a ldw assembly instruction. If I do the same memory-mapped read using the macros supplied with NIOS II, the compiler uses a ldwio. What gives? Did Altera hack their compiler to explicitly not support a struct-based interface for memory-mapped IO? 

 --Jordan

[Altera_Forum]

Hi Jordan, 

 > However, every time I read from a memory address through the old struct interface of NIOS I, > the compiler uses a ldw assembly instruction.  This is correct behavior.  > If I do the same memory-mapped read using the macros supplied with NIOS II, the compiler > uses a ldwio.  If you're referring to the IORD/WR macros, this is correct.  > What gives?  The compiler doesn't know anything about your data cache. If you don't implement a data cache, then ldw/ldwio behave identically. However, if you do implement a data cache you need to do one of two things:  1. Use the IO macros, which will explicitly use ldxio, or, 2. Make your structs (describing registers) volatile, and use -mno-cache-volatile.  The volatile keyword in no sufficient -- it only tells the compiler that it must perform memory read/write on each object reference (rather than storing it in a register). But it doesn't tell the compiler anything about _how_ to access the object -- that's where the compiler flag comes in :-)  Hope this helps.  Regards, --Scott

[Altera_Forum]

Ah, I didn't know about the compiler flag. 

 Thanks, that does help.  --Jordan