AVR-Chat

1111-111r rrrr-Xbbb

Skip if Bit in Register is Set.

FFFFh means:

Skip over the next instruction if bit 7 of Register 31 (1Fh) is set.

It means that leaving the rest of the memory programmed as FFFFh, the AVR
will be running over the entire FFFFh area instruction by instruction if
Reg31 bit 7 is off. If such bit is on, it will run over the same area but
jumping one instruction after executing the previous, so the program counter
will execute only instructions addresses of multiple of 4 bytes starting at
the first FFFF found.

It means that if by some reason a block of FFFFh is found in a sequence of
program, if such bit 7 is off nothing bad will happens, since the AVR will
go completely over the FFFFh without any problem and will execute the first
valid instruction after the block.

But if bit 7 is on, and the FFFFh block size (byte count) not being multiple
of 4, then the program counter may exit the FFFFh block skiping over the
first valid instruction after the block.  The reason is that when such bit 7
is on, the counter jumps one instruction (2 bytes) and execute next, jumps
one instruction and executes next. The "next" (jumped) can be the first
valid after the block.

Wagner Lipnharski - email:  wagner@ustr.net
UST Research Inc. - Development Director
http://www.ustr.net - Orlando Florida 32837
Licensed Consultant Atmel AVR _/_/_/_/_/_/
March 5, 2004 10:34 am



----- Original Message -----
From: "David VanHorn" <dvanhorn@cedar.net>
To: <AVR-Chat@yahoogroups.com>
Sent: Thursday, March 04, 2004 5:01 PM
Subject: [AVR-Chat] Opcodes


>
> Odd question: I know that 0000h is a nop, but what does FFFFh do?
>
> When one leaves areas of memory unprogrammed, this makes a difference, if
the program jumps there.



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