AND

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AND
Instruction AND[S]
Function Logically AND two values
Category Data processing
ARM family All
Notes -

Contents

AND[S]

AND will perform a logical AND between the two operands, placing the result in the destination register; this is useful for masking the bits you wish to work on.

Operand 1 is a register, operand 2 can be a register, shifted register, or an immediate value (which may be shifted).

If the S bit is set (ANDS), the N and Z flags are set according to the result, the C flag is set according to the shift (if used), and the V flag remains unaltered.

AND is useful for masking bitfields in order to extract only the part that is necessary. For example, if the lower two bits contain flags, ANDing with 3 will keep those flag bits and discard any other data.

Syntax

  AND<suffix>  <dest>, <op 1>, <op 2>

Function

  dest = op_1 AND op_2

AND sets the result bit only if both of the input bits are set.

Truth table:

op_1 bit op_2 bit Result bit
0 0 0
0 1 0
1 0 0
1 1 1

Example

  AND  R0, R0, #3  ; Keep bits zero and one of R0, discard the rest.
  ANDS R0, R0, #3  ; As above, but also updates status flags

Concept code

Demonstrating AND. We set up and pass a value in R1 (B%) and a mask in R2 (C%). The assembly code is called, which places the result in R0, which is thus returned and displayed.

  DIM code% 16
  P% = code%
  [ OPT    2
  
    AND    R0, R1, R2   ; R0 = R1 AND R2
  
    MOV    PC, R14
  ]
  
  A% = 0
  B% = 255     : REM Value to mask
  C% = %1111   : REM Bitmask
  
  PRINT "Result is " + STR$(USR(code%))

Demonstrating ANDS. We will take a value in R1 (B%), a mask in R2 (C%), and perform the operation. If the result is non-zero, we say so and print what the result is. If the result is zero, we say so. As it is non-trivial to pass flags back to BASIC on all types of ARM, all of the output is performed directly within the assembler code.

  DIM code% 128
  FOR loop% = 0 TO 2 STEP 2
  P% = code%
  [ OPT    loop%
  
    ; Print introduction message
    ADR    R0, resultis
    SWI    "OS_Write0"
  
    ; Perform the operation and test result
    ANDS   R0, R1, R2'
    BEQ    ifequal
  
    ; if result is non-zero, we do this
    ADR    R1, resultis   ; reuse this memory
    MOV    R2, 11
    SWI    "OS_ConvertCardinal4"
  
    ADR    R0, notequal
    SWI    "OS_Write0"
    ADR    R0, resultis
    SWI    "OS_Write0"
    MOV    PC, R14
    
  .ifequal
    ADR    R0, isequal
    SWI    "OS_Write0"
    MOV    PC, R14
    
  .resultis
    EQUS   "Result is " + CHR$(0)
    ALIGN
    
  .notequal
    EQUS   "non-zero, is: " + CHR$(0)
    ALIGN
  
  .isequal
    EQUS   "zero" + CHR$(0)
    ALIGN
  ]
  NEXT
  
  A% = 0
  B% = 255     : REM Value to mask
  C% = %1111   : REM Bitmask
  CALL code%

To see these in action, the simplest method is to alter B% to 240 (%11110000).

Technical

The instruction bit pattern is as follows:

31 - 28 27 26 25 24 - 21 20 19 - 16 15 - 12 11 - 0
condition 0 0 I 0 0 0 0 S op_1 dest op_2/shift

Note: If the I bit is zero, and bits 4 and 7 are both one (with bits 5,6 zero), the instruction is MUL, not AND.

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