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Re: [avr-libc-dev] Port and Memory Mapped Register Definitions

From: Theodore A. Roth
Subject: Re: [avr-libc-dev] Port and Memory Mapped Register Definitions
Date: Fri, 27 Sep 2002 10:30:17 -0700 (PDT)

Hi Bill,

On Fri, 27 Sep 2002, Bill Somerville wrote:

:) Hi
:) I don't see any activity on the list, so I hope someone is there.

I'm here, but silently brooding over my next evil attempt at world

:) Four points about the library:
:) 1)
:) Is there any reason why the above should not be defined along the
:) following lines:
:) #define REG_ADDR 0xnn
:) #define REG _SFR_IO8(REG_ADDR)
:) rather than the direct:
:) #define REG _SFR_IO8(0xnn)
:) The reason being to enable the REG_ADDR macro to be passed as a C++
:) template parameter to a template class like:
:) template< const uint8_t Port_addr, const uint8_t Ddr_addr >
:) class IOPort
:) {
:)    public:
:)       IOPort( uint8_t direction_flags )
:)       {
:)           *( volatile uint8_t * const )Ddr_addr = direction_flags;
:)       }
:)       IOPort& operator=( uint8_t value )
:)       {
:)          *( volatile uint8_t * const )Port_addr = value;
:)       }
:)       operator volatile uint8_t()
:)       {
:)          return *( volatile uint8_t * const )Port_addr;
:)       }
:) };
:) Used like:
:) IOPort< PORTA, DDRA > port_a( 0xff );
:) port_a = port_a & 0xf0 | ~i & 0x0f; // put low nibble of i on port a
:) This then allows constructs like mutex guarded ports to be easily
:) defined.

If you are going to do that, I think you should just disable the _SFR_*
macros complete since you're replacing their functionality (mostly). I
think you'd still want to use them to get the mmio offsets though. Read


I'm guessing you want to define _SFR_ASM_COMPAT in all your source
(-D_SFR_ASM_COMPAT on the gcc/g++ command line) which should disable the
use of _MMIO_{BYTE,WORD} macros, but still give you the correct (raw) mmio
addrs which you can cast as you do in your template.

OT: What's the code space overhead of using the templates to do this? I'm
not a C++ guy, so I'm curious.

:) 2)
:) It appears that outb() and inb() always use memory mapped i/o ports
:) rather than registers if possible - is this true and if so why?

Hmmm. I don't have a clue. Marek reimplemented these using the _SFR_*
mechanism, so that might have something to do with it.

:) 3)
:) The setjmp.h header file is not C++ compatible (no extern "C++"
:) declaration), is this deliberate?

I don't know the history on this, but I don't see any harm in fixing it.
I'll fix it, but I would like to know if the jmp_buf struct definition
needs to be protected too?

:) 4)
:) Given that g++ generates avr code and gcrt1.S is now C++ compatible, it
:) would be nice to have a very basic C++ library with the various
:) std::new, std::delete, and std::set_new_handler fns defined, or at least
:) the nothrow versions.

I assume you're talking about a "watered down" version of libstdc++ for
avr? I'm not against it, but I don't have the know-how to do. I vaguely
remember a discussion of this on avr-gcc-list a while back (Feb-2002
archives show some discussion).

Due to the small memory available for the avrs, this would have to be very
minimalistic. If you'd like to offer up some code, we can consider it for

Ted Roth

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