{"id":185683,"date":"2025-01-22T20:16:17","date_gmt":"2025-01-22T20:16:17","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=185683"},"modified":"2025-01-22T20:16:19","modified_gmt":"2025-01-22T20:16:19","slug":"use-ll-sc-instructions-to-construct-an-atomic-compare-and-swap-instruction-cas-r1-r2-l","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/01\/22\/use-ll-sc-instructions-to-construct-an-atomic-compare-and-swap-instruction-cas-r1-r2-l\/","title":{"rendered":"Use LL\/SC instructions to construct an atomic compare and swap instruction \u201cCAS R1, R2, L\u201d"},"content":{"rendered":"\n<p>Use LL\/SC instructions to construct an atomic compare and swap instruction \u201cCAS R1, R2, L\u201d which tests whether data in memory location L is equal to that in R1. If they are equal, write the value in R2 to L, and copy R1 to R2. Otherwise, nothing is done and CAS returns. For example, if initially R1=5, R2=10, L=5, after CAS we have R1=5, R2=5, and L=10. If initially R1=7, R2=10, L=5, nothing changes after CAS. Show your answer in assembly code, and annotate what each instruction does. Keep as few instructions between LL and SC as possible.<\/p>\n\n\n\n<p><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-ast-global-color-6-color\"><strong>The Correct Answer and Explanation is :<\/strong><\/mark><\/p>\n\n\n\n<p>To construct the atomic compare and swap (CAS) instruction using LL\/SC (Load-Linked\/Store-Conditional) instructions, we can use the following assembly code. LL\/SC is typically used to implement atomic operations like CAS, ensuring that the memory operation is performed atomically and is only successful if the memory location has not been modified by another process or thread.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">CAS Assembly Code (using LL\/SC):<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>CAS:\n    LL    R3, L         ; Load linked value from memory location L into R3\n    CMP   R1, R3        ; Compare R1 (the expected value) with R3 (value in memory)\n    BNE   CAS_DONE      ; If R1 \u2260 R3, branch to CAS_DONE (nothing to do)\n\n    SC    R4, R2, L     ; Store conditional: attempt to store R2 to L\n    BEQ   CAS_SUCCESS   ; If store conditional succeeded (R4 = 0), branch to CAS_SUCCESS\n\nCAS_DONE:\n    ; If CAS is not successful, do nothing, return.\n\nCAS_SUCCESS:\n    MOV   R1, R3        ; Move the value from R3 (the old value of L) to R1\n    MOV   R2, R3        ; Move the value from R3 to R2\n    RET                 ; Return from the CAS function\n<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">Explanation of Each Instruction:<\/h3>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>LL R3, L<\/strong>: The <code>LL<\/code> instruction loads the value from memory location <code>L<\/code> into register <code>R3<\/code>. This instruction is part of the atomic pair used in LL\/SC operations. It effectively marks the start of the atomic operation.<\/li>\n\n\n\n<li><strong>CMP R1, R3<\/strong>: The <code>CMP<\/code> instruction compares the value in <code>R1<\/code> (which is the expected value) with the value in <code>R3<\/code> (the current value of <code>L<\/code>). If these values are different, the CAS operation cannot proceed.<\/li>\n\n\n\n<li><strong>BNE CAS_DONE<\/strong>: If the comparison between <code>R1<\/code> and <code>R3<\/code> shows they are not equal (<code>BNE<\/code> stands for &#8220;Branch if Not Equal&#8221;), the function branches to <code>CAS_DONE<\/code> without making any changes, since the value in memory doesn&#8217;t match the expected value.<\/li>\n\n\n\n<li><strong>SC R4, R2, L<\/strong>: If the comparison succeeds (i.e., <code>R1<\/code> equals <code>R3<\/code>), the <code>SC<\/code> (Store-Conditional) instruction attempts to store the value in <code>R2<\/code> to memory location <code>L<\/code>. If no other processor or thread has modified <code>L<\/code> since the <code>LL<\/code> instruction, the store is successful, and <code>R4<\/code> will be set to 0.<\/li>\n\n\n\n<li><strong>BEQ CAS_SUCCESS<\/strong>: If the <code>SC<\/code> instruction succeeded (<code>R4 == 0<\/code>), the function branches to <code>CAS_SUCCESS<\/code>, where the actual swap and return will take place.<\/li>\n\n\n\n<li><strong>MOV R1, R3<\/strong>: Once the store succeeds, the old value of <code>L<\/code> (which is in <code>R3<\/code>) is moved to <code>R1<\/code>, so that <code>R1<\/code> holds the original value of <code>L<\/code>.<\/li>\n\n\n\n<li><strong>MOV R2, R3<\/strong>: Similarly, the old value of <code>L<\/code> is moved to <code>R2<\/code>, ensuring that <code>R2<\/code> gets the old value from memory.<\/li>\n\n\n\n<li><strong>RET<\/strong>: The function returns after successfully completing the CAS operation.<\/li>\n<\/ol>\n\n\n\n<h3 class=\"wp-block-heading\">CAS Behavior:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>If the memory value at <code>L<\/code> is equal to <code>R1<\/code>, the CAS operation stores <code>R2<\/code> into <code>L<\/code>, updates <code>R1<\/code> and <code>R2<\/code> with the old value of <code>L<\/code>, and the operation completes successfully.<\/li>\n\n\n\n<li>If the memory value at <code>L<\/code> is different from <code>R1<\/code>, no changes are made, and the function returns without doing anything.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Atomicity:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>The <code>LL<\/code> and <code>SC<\/code> instructions ensure that the entire compare-and-swap operation is atomic. If any other process modifies the memory location between the <code>LL<\/code> and <code>SC<\/code>, the store fails, and the operation does not proceed.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Use LL\/SC instructions to construct an atomic compare and swap instruction \u201cCAS R1, R2, L\u201d which tests whether data in memory location L is equal to that in R1. If they are equal, write the value in R2 to L, and copy R1 to R2. Otherwise, nothing is done and CAS returns. For example, if [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[25],"tags":[],"class_list":["post-185683","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/185683","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/comments?post=185683"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/185683\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=185683"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=185683"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=185683"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}