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

[E. Weddington]

On 8 May 2003 at 14:44, Wallace White wrote:

> 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.
> 

There's been a lot of discussion and it seems the most efficient way 
is to use a union.

#include <inttypes.h>

typedef struct
{
        unsigned char lo;
        unsigned char hi;
} hilow_t;

typedef union
{
        uint16_t word;
        hilow_t byte;
} mytype ;

mytype encPosDetentsNow;

....
{
        ....
        encPosDetentsNow.word = 0xFFFF; 
        UDR = encPosDetentsNow.byte.hi;
        ....
}

See what you get with this.

Eric