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Re: [avr-gcc-list] strange c

From: Rich Neswold
Subject: Re: [avr-gcc-list] strange c
Date: Tue, 18 Dec 2001 12:09:40 -0600
User-agent: Mutt/1.2.5i

Wasn't it 18-Dec-2001, at 02:24PM, when Torsten Hahn said:
> im a bit new to low-level c, but what does e.g 1<<TOIE1 do ?  From the
> languege reference i know the shift operator to use like value<<1 but not
> the other way around.

    1 << n

Creates a value that has the n'th bit set (the lowest bit is the 0th bit.)

    1 << 0      gives   0x01
    1 << 1      gives   0x02
    1 << 2      gives   0x04

The IO headers files in avr-libc define all the bits in the I/O registers
in terms of their position (i.e. 0-7 values). This works well when using
the cbi()/sbi() macros. If you want to load several bits at the same time
(using outp()), you could use the BV() macro, which computes the formula (1
<< arg). By ORing together several BV() instances, you build up a value
with several bits:

    outp(BV(first_bit_of_interest) | BV(second_bit), target_register);

The previous statement will generate one line of assembly (as long as
"target_register" is an I/O register.)

Hope this helps...

 Rich Neswold
 efax: 1.240.536.7092
  web: www.enteract.com/~rneswold/
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