{"id":217717,"date":"2025-05-22T11:24:50","date_gmt":"2025-05-22T11:24:50","guid":{"rendered":"https:\/\/learnexams.com\/blog\/?p=217717"},"modified":"2025-05-22T11:24:52","modified_gmt":"2025-05-22T11:24:52","slug":"suppose-the-arm-6-has-the-following-initial-register-and-memory-contents","status":"publish","type":"post","link":"https:\/\/www.learnexams.com\/blog\/2025\/05\/22\/suppose-the-arm-6-has-the-following-initial-register-and-memory-contents\/","title":{"rendered":"Suppose the ARM 6 has the following initial register and memory contents"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Suppose the ARM 6 has the following initial register and memory contents: r1=00000000; r2-87654321; r3-a05877f9; NZCV 0000.identify the new contents of register or flag that is changed by execution of the following instructions: (a) add r1, r2, r3 (b) adds r1, r3, r3<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong><mark style=\"background-color:rgba(0, 0, 0, 0)\" class=\"has-inline-color has-ast-global-color-1-color\">The Correct Answer and Explanation is:<\/mark><\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Let&#8217;s examine the execution of the two ARM instructions given the initial register contents:<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Initial Register and Flag Values:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>r1 = 0x00000000<\/code><\/li>\n\n\n\n<li><code>r2 = 0x87654321<\/code><\/li>\n\n\n\n<li><code>r3 = 0xA05877F9<\/code><\/li>\n\n\n\n<li><code>NZCV (Negative, Zero, Carry, Overflow) = 0000<\/code><\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">(a) <code>ADD r1, r2, r3<\/code><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">This instruction performs:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>r1 = r2 + r3<\/code><\/pre>\n\n\n\n<h4 class=\"wp-block-heading\">Calculation:<\/h4>\n\n\n\n<pre class=\"wp-block-code\"><code>r2 = 0x87654321\nr3 = 0xA05877F9\n\nr2 + r3 = 0x87654321 + 0xA05877F9\n         = 0x277DBB1A (with carry-out ignored because it's ADD, not ADDS)<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Since we&#8217;re using <strong>ADD<\/strong>, the <strong>condition flags are not affected<\/strong>.<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">New Register Content:<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>r1 = 0x277DBB1A<\/code><\/li>\n\n\n\n<li><strong>Flags unchanged (NZCV = 0000)<\/strong><\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">(b) <code>ADDS r1, r3, r3<\/code><\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">This instruction performs:<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>r1 = r3 + r3<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">and <strong>updates the flags<\/strong> (since it is <code>ADDS<\/code>).<\/p>\n\n\n\n<h4 class=\"wp-block-heading\">Calculation:<\/h4>\n\n\n\n<pre class=\"wp-block-code\"><code>r3 = 0xA05877F9\n\nr3 + r3 = 0xA05877F9 + 0xA05877F9\n         = 0x140B0EFF2\n\nThis is a 33-bit result. ARM registers are 32-bit, so the result stored in r1 will be:\nr1 = 0x40B0EFF2 (lower 32 bits)\n\nCarry-out occurred, so **C flag = 1**<\/code><\/pre>\n\n\n\n<p class=\"wp-block-paragraph\">Now let\u2019s evaluate flags:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>N (Negative)<\/strong>: 0 (MSB of result is 0)<\/li>\n\n\n\n<li><strong>Z (Zero)<\/strong>: 0 (Result is not zero)<\/li>\n\n\n\n<li><strong>C (Carry)<\/strong>: 1 (because the result overflowed 32 bits)<\/li>\n\n\n\n<li><strong>V (Overflow)<\/strong>: 1 (because adding two negative numbers gave a positive result)<\/li>\n<\/ul>\n\n\n\n<h4 class=\"wp-block-heading\">New Register and Flags:<\/h4>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>r1 = 0x40B0EFF2<\/code><\/li>\n\n\n\n<li><code>NZCV = 0011<\/code> \u2192 N=0, Z=0, C=1, V=1<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">Summary of Changes:<\/h3>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>Instruction<\/th><th>New <code>r1<\/code> Value<\/th><th>Flags Affected<\/th><th>New NZCV<\/th><\/tr><\/thead><tbody><tr><td><code>ADD r1, r2, r3<\/code><\/td><td><code>0x277DBB1A<\/code><\/td><td>No<\/td><td>Unchanged (0000)<\/td><\/tr><tr><td><code>ADDS r1, r3, r3<\/code><\/td><td><code>0x40B0EFF2<\/code><\/td><td>Yes<\/td><td><code>0011<\/code> (C=1, V=1)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h3 class=\"wp-block-heading\">300-Word Explanation:<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The ARM instructions <code>ADD<\/code> and <code>ADDS<\/code> both perform arithmetic addition, but they differ significantly in how they affect the condition flags (NZCV). Let&#8217;s break down the effects step-by-step using the given initial values:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>ADD r1, r2, r3<\/strong> simply computes the sum of <code>r2<\/code> and <code>r3<\/code> and stores the result in <code>r1<\/code>. The binary sum of <code>0x87654321<\/code> and <code>0xA05877F9<\/code> results in <code>0x127DBB1A2<\/code>, which is more than 32 bits. Since <code>ADD<\/code> does not update the condition flags, even though an arithmetic carry occurs, it is ignored for flag purposes. The final result stored in <code>r1<\/code> is the lower 32 bits: <code>0x277DBB1A<\/code>. The flags remain unchanged.<\/li>\n\n\n\n<li><strong>ADDS r1, r3, r3<\/strong> performs an addition of <code>r3<\/code> with itself and stores the result in <code>r1<\/code>, but unlike <code>ADD<\/code>, it updates the condition flags. The sum of <code>0xA05877F9 + 0xA05877F9<\/code> equals <code>0x140B0EFF2<\/code>. Only the lower 32 bits (<code>0x40B0EFF2<\/code>) are stored in <code>r1<\/code>. Because the result exceeds 32 bits, a <strong>carry<\/strong> occurs, so the <strong>C flag is set<\/strong>. Moreover, adding two negative numbers (as the sign bit of <code>r3<\/code> is 1) resulting in a positive value causes a <strong>signed overflow<\/strong>, so the <strong>V flag is also set<\/strong>. The <strong>N flag<\/strong> is 0 (most significant bit of the result is 0), and <strong>Z<\/strong> is 0 (result is not zero).<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">In summary, <code>ADD<\/code> only changes the destination register, while <code>ADDS<\/code> changes both the destination register and the status flags. This distinction is critical in ARM assembly, especially for conditional execution based on result flags<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><img decoding=\"async\" src=\"https:\/\/learnexams.com\/blog\/wp-content\/uploads\/2025\/05\/image-50.png\" alt=\"\" class=\"wp-image-217718\"\/><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Suppose the ARM 6 has the following initial register and memory contents: r1=00000000; r2-87654321; r3-a05877f9; NZCV 0000.identify the new contents of register or flag that is changed by execution of the following instructions: (a) add r1, r2, r3 (b) adds r1, r3, r3 The Correct Answer and Explanation is: Let&#8217;s examine the execution of the [&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-217717","post","type-post","status-publish","format-standard","hentry","category-exams-certification"],"_links":{"self":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/217717","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=217717"}],"version-history":[{"count":0,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/posts\/217717\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/media?parent=217717"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/categories?post=217717"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.learnexams.com\/blog\/wp-json\/wp\/v2\/tags?post=217717"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}