The x86-64 architecture inherited the parity flag (PF) from the x86-32, which in turn inherited it from the 8080, which inherited it from the 8008, which implemented it because it was the processor for the Datapoint 2200 serial terminal.<\/p>\n
The parity flag also has a secondary purpose of being a place for the It turns out that the Windows debugging engine for x86-64 had a bug where it reported the parity flag as the opposite of what it actually is. When the parity flag was set, it said “po” instead of “pe”, and vice versa.<\/p>\n The fact that this went unreported for over two decades tells you that nobody cares about debugging the parity flag.<\/p>\n A fix has gone in. We’ll see if it makes it out before this article gets posted.<\/p>\n","protected":false},"excerpt":{"rendered":" Reported incorrectly since the day it was written.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[2],"class_list":["post-112334","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":" Reported incorrectly since the day it was written.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112334","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=112334"}],"version-history":[{"count":1,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112334\/revisions"}],"predecessor-version":[{"id":112336,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/112334\/revisions\/112336"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=112334"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=112334"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=112334"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}FXAM<\/code> (x87) and UCOMISD<\/code> (SSE) instructions to record the results of floating point comparisons. You can still entice compilers into checking the parity flag by checking a value for NaN or performing a floating point equality or inequality comparison (because NaN always fails equality and inequality comparison).<\/p>\n