#include <avr/io.h>
#include <inttypes.h>
#include <avr/interrupt.h>     // задает макросы sei() , cli()

#define  KeyMask (1<<Btn1)|(1<<Btn2)

volatile register int8_t a asm ("r16");

int main (void)
{
sei();
a |= 1;

while(1)
{
  asm ("nop ");
  if (~(PINB)&(KeyMask))
        {
        PORTB=KeyMask;
        a |= 0x02;
        }
}
}

+00000011:   9478        SEI                      Global Interrupt Enable
+00000012:   E02C        LDI     R18,0x0C         Load immediate
+00000013:   0000        NOP                      No operation
+00000014:   B386        IN      R24,0x16         In from I/O location
+00000015:   E090        LDI     R25,0x00         Load immediate
+00000016:   9580        COM     R24              One's complement
+00000017:   9590        COM     R25              One's complement
+00000018:   708C        ANDI    R24,0x0C         Logical AND with immediate
+00000019:   7090        ANDI    R25,0x00         Logical AND with immediate
+0000001A:   2B89        OR      R24,R25          Logical OR
+0000001B:   F3B9        BREQ    PC-0x08          Branch if equal
+0000001C:   BB28        OUT     0x18,R18         Out to I/O location
+0000001D:   CFF5        RJMP    PC-0x000A        Relative jump