{"id":1848,"date":"2016-04-09T00:01:15","date_gmt":"2016-04-09T07:01:15","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=1848"},"modified":"2016-04-16T07:41:48","modified_gmt":"2016-04-16T14:41:48","slug":"reading-bits-and-resetting-bits","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=1848","title":{"rendered":"Reading Bits and Resetting Bits"},"content":{"rendered":"

Once you break out the tools, bit manipulation in C can be a fascinating and useful thing. Setting a bit, covered in last week’s Lesson<\/a>, is a big first step. Once you’ve done that, the functions to read and set a bit fall into place naturally.
\n
\nThe goal in each function is to access a specific bit. From last week’s code snippet, the following statement is used:<\/p>\n

bit = 1 << bit;<\/code><\/pre>\n
Variable bit<\/code> represents the bit position in a byte, word, or long word. Position 0 is at the far right. Position 1 comes next, and so on. The function sets bit 0 to the value 1. The left shift operator, <<<\/code>, shifts that value left to the proper bit position.<\/p>\n
To read a bit at a specific position, you must mask out all other bits in the value. The operator that assists in that process is the bitwise &<\/code> (and). After you mask out all the other bits, the value that remains is either zero or some other value. These two conditions evaluate to FALSE and TRUE in the C language, which tells you whether or not the bit was set.<\/p>\n
In Figure 1, you see binary value 1 shifted twice to bit position 2, as in bit = 1 << bit;<\/code> where bit<\/code> equals 2.<\/p>\n
\"Figure
Figure 1. Variable bit<\/code> is set to binary position 2.<\/p><\/div>\n
Once the bit is positioned, an &<\/code> (and) mask eliminates all other bits but position 2 in the original value. The result is either 0 or 4 depending on whether the bit was off or on, respectively. Figure 2 illustrates how this operation works.<\/p>\n
\"Figure
Figure 2. How an &<\/code> (and) mask works.<\/p><\/div>\n
The function I wrote to read bit status is called bit_test()<\/em>. It uses the same arguments as bit_set()<\/em> from last week’s Lesson:<\/p>\n
\r\nint bit_test(char bit, char byte)\r\n{\r\n    bit = 1 << bit;\r\n    return(bit & byte);\r\n}<\/pre>\n
The first statement sets the proper position for variable bit<\/code>. The second statement uses the &<\/code> (and) mask to isolate that bit and return the value.<\/p>\n
The value returned by bit_test()<\/em> is either true, meaning the bit was set, or false when the bit is zero.<\/p>\n
The final bit manipulation function is bit_reset()<\/em>, which resets a bit’s value to zero. This function uses the same arguments as bit_set()<\/em> and bit_test()<\/em>, which provides consistency.<\/p>\n
To reset a bit, the bit_reset()<\/em> function uses both the bitwise ^<\/code> (exclusive or) and &<\/code> (and) operators. These operators help to isolate the individual bit and set only that bit to 0.<\/p>\n
Here is the function:<\/p>\n
\r\nvoid bit_reset(char bit, char *byte)\r\n{\r\n    bit = 1 << bit;\r\n    bit ^= 0xff;\r\n    *byte = *byte & bit;\r\n}<\/pre>\n
The statement bit ^= 0xff;<\/code> inverts the bits. The net effect is to mask off all bits save for the one to reset, as illustrated in Figure 3.<\/p>\n
\"Figure
Figure 3. The ^<\/code> (xor) operator inverts the bit field, creating a mask.<\/p><\/div>\n
Once the mask is set, the &<\/code> (and) operator filters through every bit but the one to reset. The end result is that only one bit is reset.<\/p>\n
In next week’s Lesson<\/a>, I put all three functions to work in a demo program that manipulates bits in a char<\/em> value.<\/p>\n","protected":false},"excerpt":{"rendered":"
More bit manipulation fun! Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-1848","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1848","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1848"}],"version-history":[{"count":8,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1848\/revisions"}],"predecessor-version":[{"id":1893,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1848\/revisions\/1893"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1848"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1848"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1848"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}