Difference between revisions of "Logical right shift"
(Added universal routines.) |
m (Fixed broken formatting.) |
||
| Line 21: | Line 21: | ||
|- style="vertical-align:top" | |- style="vertical-align:top" | ||
| Associated keywords | | Associated keywords | ||
| − | | colspan="2" | <code>[[Left shift|<<]]</code>, <code>[[Arithmetic right shift|>>]]</code>, <code>[[AND]]</code>, <code>[[DIV]]</code>, <code>[[EOR]]</code | + | | colspan="2" | <code>[[Left shift|<<]]</code>, <code>[[Arithmetic right shift|>>]]</code>, <code>[[AND]]</code>, <code>[[DIV]]</code>, <code>[[EOR]]</code>, <code>[[NOT]]</code>, <code>[[OR]]</code> |
|} | |} | ||
== Description == | == Description == | ||
| − | |||
<code>>>></code> accepts two integer values, and returns the first value | <code>>>></code> accepts two integer values, and returns the first value | ||
''right-shifted'' by the number of binary places given in the second | ''right-shifted'' by the number of binary places given in the second | ||
operand. For instance, in the statement | operand. For instance, in the statement | ||
| − | + | PRINT ~&87654321 >>> 4 | |
the binary digits of &87654321, 10000111011001010100001100100001, are moved | the binary digits of &87654321, 10000111011001010100001100100001, are moved | ||
| − | four places to the right in the register. | + | 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) | significant bits (0001) are discarded, and the sixteens' place (bit 4) | ||
| − | becomes the units' place (bit 0) of the returned value. | + | 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 | ''logical shift'', zeroes are shifted in at the top, so that bits 28 to 31 | ||
| − | of the result are all 0s. | + | 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 | but if we consider the shift as a step-by-step calculation, it may be | ||
represented as: | represented as: | ||
| − | + | Operand 10000111011001010100001100100001 | |
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
\ | \ | ||
| − | 4th shift 00001000011101100101010000110010 | + | 1st shift 01000011101100101010000110010000 → |
| + | \ | ||
| + | 2nd shift 00100001110110010101000011001000 → | ||
| + | \ | ||
| + | 3rd shift 00010000111011001010100001100100 → | ||
| + | \ | ||
| + | 4th shift 00001000011101100101010000110010 → | ||
The output from the statement is: | The output from the statement is: | ||
| − | + | >PRINT ~&87654321 >>> 4 | |
| − | + | 8765432 | |
== Notes == | == Notes == | ||
| − | |||
<code>>>></code> takes the two's complement of a negative shift distance. | <code>>>></code> takes the two's complement of a negative shift distance. | ||
On ARM processors, a shift distance between 32 and 255 inclusive returns | On ARM processors, a shift distance between 32 and 255 inclusive returns | ||
zero; any larger, or negative, distance <code>N%</code> has the same effect | zero; any larger, or negative, distance <code>N%</code> has the same effect | ||
| − | as <code>N% AND 255</CODE>. | + | as <code>N% AND 255</CODE>. BASIC on other architectures may give different |
results when the second operand is more than 31 or less than 0. | results when the second operand is more than 31 or less than 0. | ||
As a Group 5 operator <code>>>></code> does not associate with other Group 5 | As a Group 5 operator <code>>>></code> does not associate with other Group 5 | ||
operators, so parentheses must be used when an operand has such an operator | operators, so parentheses must be used when an operand has such an operator | ||
| − | on either side. | + | on either side. For example: |
| − | + | IF (dword% >>> 24) = 1 THEN ... | |
See <code>[[=]]</code> for more details. | See <code>[[=]]</code> for more details. | ||
Latest revision as of 16:26, 15 January 2024
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)
