Bit Manipulation

Bit Manipulation

When programming in assembly language, it is often helpful to manipulate individual bits.

Shift and Rotate

• ROR and ROL rotate 9 bits (8 bits plus carry flag) to the right or left
• LSL and LSR: logical shifts are like the rotates except the values don't wrap around.
  • The least significant bit is cleared when doing a LSL.
  • The most significant bit is cleared when doing a LSR.
• ASR: Arithmetic shift is like LSR except it retains the value of the most significant bit.

Bit Manipulation

• SBI port, bit: Set bit in port

      sbi  PORTB, 7

• The above is equivalent to:

      in  r16, PORTB
      ori r16, 0b10000000
      out PORTB, r16

• CBI: Clear bit in port
• CBI and SBI only operates on one bit at a time.
• CBR Rd, mask: Clear Bit in Register (R16-R31)
  • mask — contains 1's in positions to be cleared.

      cbr  r16, 0x0f      ; Clears the four least significant bits

• CLR Rd — clears all bits in Rd.
• SER Rd — sets all bits in Rd (R16-R31).
  • To do the equivalent on R0-R15, we could do this:

      clr r0
      com r0  ; Perform 2's complement on R0

• SBIC PORT, bit — skip if bit clear
  • The following will continue to loop until the most significant bit is cleared:

poll:
      sbic PIND, 7  ; skips next instruction if bit 7 is cleared
      rjmp poll

• SBIS PORT, bit — skip if bit set
• SBIS only works on the first 32 I/O ports (which includes PORTA - PORTD).