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Re: [avr-gcc-list] 24 bits (u)ints

From: Georg-Johann Lay
Subject: Re: [avr-gcc-list] 24 bits (u)ints
Date: Wed, 30 Nov 2016 14:14:23 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

On 30.11.2016 08:40, Diogo Martins Silva wrote:
Hello all.

The avr-gcc wiki (https://gcc.gnu.org/wiki/avr-gcc) lists 24 bits
(u)ints as an extension since version 4.7. How do I use them?



You can use it basically like any other integer type, for example:

__uint24 mul24 (unsigned char a, unsigned int b)
    return (__uint24) a * b;

__int24 get_val24 (const __flash __int24 vals[], unsigned char pos)
    return vals[pos];

For a MCU with MUL, this gives

        mul r24,r22
        movw r18,r0
        mul r24,r23
        add r19,r0
        mov r20,r1
        clr __zero_reg__
        adc r20,__zero_reg__
        mov r24,r20
        movw r22,r18

        ldi r18,lo8(3)
        mul r22,r18
        add r24,r0
        adc r25,r1
        clr __zero_reg__
        movw r30,r24
        lpm r22,Z+
        lpm r23,Z+
        lpm r24,Z

When you are passing a 24-bit value to a variadic function like printf, you have to cast it to (un)signed long by hand; avr-gcc does not automatically promote such types to 32-bit types. printf et al. have no format specifiers for 24-bit types.


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