Logical right shift

Logical right shift, >>>, is a bitwise operator to shift a binary value to the right, without sign extension.

>>>

Availability	BASIC V
Syntax	BASIC V	<num-var> = <numeric> >>> <numeric>
Token (hex)	BASIC V	3E 3E 3E (operator)
Description	BASIC V	Returns the first operand, with the bits of its binary representation shifted toward the least significant end by the number of places specified in the second operand, and zeroes shifted in at the most significant end.
Associated keywords	<<, >>, AND, DIV, EOR, NOT, OR

Description

>>> accepts two integer values, and returns the first value right-shifted by the number of binary places given in the second operand. For instance, in the statement

   PRINT ~&87654321 >>> 4

the binary digits of &87654321, 10000111011001010100001100100001, are moved four places to the right in the register. In this case, the four least significant bits (0001) are discarded, and the sixteens' place (bit 4) becomes the units' place (bit 0) of the returned value. As it is a logical shift, zeroes are shifted in at the top, so that bits 28 to 31 of the result are all 0s. BASIC performs the shift in a single instruction, but if we consider the shift as a step-by-step calculation, it may be represented as:

   Operand    10000111011001010100001100100001
               \
   1st shift  01000011101100101010000110010000 →
                \
   2nd shift  00100001110110010101000011001000 →
                 \
   3rd shift  00010000111011001010100001100100 →
                  \
   4th shift  00001000011101100101010000110010 →

The output from the statement is:

   >PRINT ~&87654321 >>> 4
      8765432

Notes

>>> takes the two's complement of a negative shift distance.

On ARM processors, a shift distance between 32 and 255 inclusive returns zero; any larger, or negative, distance N% has the same effect as N% AND 255. BASIC on other architectures may give different results when the second operand is more than 31 or less than 0.

As a Group 5 operator >>> does not associate with other Group 5 operators, so parentheses must be used when an operand has such an operator on either side. For example:

   IF (dword% >>> 24) = 1 THEN ...

See = for more details.

A double greater-than sign makes the arithmetic right shift operator, >>, which duplicates the most significant bit to perform sign extension.

Compatibility

The following routines will do a binary right shift, the equivalent of >>> 1, and a binary right rotate on any BBC BASIC:

   REM Shift Right:  abcdefgh -> 0abcdefg
   DEFFNshr(A%)=((A%AND&7FFFFFFF)DIV2)OR((A%<0)AND&40000000)
   :
   REM Rotate Right: abcdefgh -> habcdefg
   DEFFNror(A%)=((A%AND&7FFFFFFF)DIV2)OR((A%<0)AND&40000000)OR(((A%AND1)>0)AND&80000000)
   :

beardo 02:54, 3 September 2011 (UTC) Jgharston (talk) 16:21, 15 January 2024 (CET)