RE: [avr-gcc-list] Time-efficient read of high byte in two-byte variable

[Ralph Mason]

I found that

INLINE u_short lpw(u_short* addr)
{
        union
        {
                u_short val;
                struct
                {
                        u_char low;
                        u_char high;
                };
        } ret;

        ret.low = (u_char) PRG_RDB(addr);
        ((u_char*)addr)++;
        ret.high = (u_char)PRG_RDB(addr);

        return  ret.val;
}

Generated ideal assembly code.  Perhaps you can use the same idea (cast it
to a strut and access the byte you want.

Let me know how it works out.

Ralph



> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden Behalf Of Wallace White
> Sent: Friday, 9 May 2003 9:45
> To: address@hidden
> Subject: [avr-gcc-list] Time-efficient read of high byte in two-byte
> variable
>
>
> What's a good, quick way to read the high byte of a two-byte
> variable in C?
>
> I am sending an int out the UART, unformatted. Here's what my tries so
> far have compiled into (my 16-bit variable is encPosDetentsNow):
>
> 872:    UDR = (unsigned char) (encPosDetentsNow >> 8);
> +00000517:   91600079    LDS     R22,0x0079       Load direct from data
> space
> +00000519:   9170007A    LDS     R23,0x007A       Load direct from data
> space
> +0000051B:   2F87        MOV     R24,R23          Copy register
> +0000051C:   2799        CLR     R25              Exclusive OR
> +0000051D:   FD87        SBRC    R24,7            Skip if bit in
> register cleared
> +0000051E:   959A        DEC     R25              Decrement
> +0000051F:   B98C        OUT     0x0C,R24         Out to I/O location
>
> The following is a little shorter:
>
> 871:   UDR = (((unsigned int) encPosDetentsNow) & 0xFF00) >> 8;
> +00000517:   91600079    LDS     R22,0x0079       Load direct from data
> space
> +00000519:   9170007A    LDS     R23,0x007A       Load direct from data
> space
> +0000051B:   2F87        MOV     R24,R23          Copy register
> +0000051C:   2799        CLR     R25              Exclusive OR
> +0000051D:   B98C        OUT     0x0C,R24         Out to I/O location
>
> What I'd really like to get would be just
>       LDS     R23,0x007A      ; load the high byte
>       OUT     0x0C,R24        ; and output it to UDR
> without going to inline assembly.
>
> This is using the current WinAVR with -O3.
>
> Thanks,
> Wallace
>
>
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