51 lines
2.7 KiB
Factor
51 lines
2.7 KiB
Factor
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USING: help.markup help.syntax math ;
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IN: math.bitwise
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ARTICLE: "math-bitfields" "Constructing bit fields"
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"Some applications, such as binary communication protocols and assemblers, need to construct integers from elaborate bit field specifications. Hand-coding this using " { $link shift } " and " { $link bitor } " results in repetitive code. A higher-level facility exists to factor out this repetition:"
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{ $subsection bitfield } ;
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ABOUT: "math-bitfields"
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HELP: bitfield
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{ $values { "values..." "a series of objects" } { "bitspec" "an array" } { "n" integer } }
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{ $description "Constructs an integer from a series of values on the stack together with a bit field specifier, which is an array whose elements have one of the following shapes:"
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{ $list
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{ { $snippet "{ constant shift }" } " - the resulting bit field is bitwise or'd with " { $snippet "constant" } " shifted to the right by " { $snippet "shift" } " bits" }
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{ { $snippet "{ word shift }" } " - the resulting bit field is bitwise or'd with " { $snippet "word" } " applied to the top of the stack; the result is shifted to the right by " { $snippet "shift" } " bits" }
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{ { $snippet "shift" } " - the resulting bit field is bitwise or'd with the top of the stack; the result is shifted to the right by " { $snippet "shift" } " bits" }
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}
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"The bit field specifier is processed left to right, so stack values should be supplied in reverse order." }
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{ $examples
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"Consider the following specification:"
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{ $list
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{ "bits 0-10 are set to the value of " { $snippet "x" } }
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{ "bits 11-14 are set to the value of " { $snippet "y" } }
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{ "bit 15 is always on" }
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{ "bits 16-20 are set to the value of " { $snippet "fooify" } " applied to " { $snippet "z" } }
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}
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"Such a bit field construction can be specified with a word like the following:"
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{ $code
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": baz-bitfield ( x y z -- n )"
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" {"
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" { fooify 16 }"
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" { 1 15 }"
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" 11"
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" 0"
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" } ;"
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}
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} ;
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HELP: bits
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{ $values { "m" integer } { "n" integer } { "m'" integer } }
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{ $description "Keep only n bits from the integer m." }
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{ $example "USING: math.bitwise prettyprint ;" "HEX: 123abcdef 16 bits .h" "cdef" } ;
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HELP: bitroll
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{ $values { "x" "an integer (input)" } { "s" "an integer (shift)" } { "w" "an integer (wrap)" } { "y" integer } }
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{ $description "Roll n by s bits to the left, wrapping around after w bits." }
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{ $examples
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{ $example "USING: math.bitwise prettyprint ;" "1 -1 32 bitroll .b" "10000000000000000000000000000000" }
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{ $example "USING: math.bitwise prettyprint ;" "HEX: ffff0000 8 32 bitroll .h" "ff0000ff" }
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} ;
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