{"id":141,"date":"2006-11-09T00:08:00","date_gmt":"2006-11-09T08:08:00","guid":{"rendered":"http:\/\/www.mccambridge.org\/blog\/2006\/11\/huzzah-functional-branching\/"},"modified":"2022-09-11T00:40:45","modified_gmt":"2022-09-11T00:40:45","slug":"huzzah-functional-branching","status":"publish","type":"post","link":"http:\/\/www.mccambridge.org\/blog\/2006\/11\/huzzah-functional-branching\/","title":{"rendered":"Huzzah!  Functional Branching!"},"content":{"rendered":"<p>Dave and I spent all of one late night implementing the first generation (unpipelined) of our 552 project, a 16-bit MIPS processor in Verilog, and then we spent a couple hours this afternoon debugging it.  Around 5:15 we ran into a snag:<\/p>\n<p>The assembler we were provided was for a slightly different instruction set architecture than the one we were required to implement.  Specifically, in that other ISA, PC-relative branch offsets assumed a least-significant bit of <code>0<\/code> and thus didn&#8217;t waste a bit in the instruction for that zero.  As a result, the next PC calculation (for that ISA) would be <code>PC < - PC + 2 + (sign ext. Immed. << 1)<\/code>.<\/p>\n<p>This is a problem, because <a href=\"http:\/\/www.cs.wisc.edu\/~markhill\/cs552\/Fall2006\/project\/isa.html\"><em>Our<\/em> ISA<\/a> is byte-addressable, and says the PC calculation should be <\/code><code>PC < - PC + 2 + sign ext. Immed.<\/code>  (See beqz instruction for an example.)<\/p>\n<p>Those of you familiar with computer architecture will recognize this as a severe problem: The assembler is generating offsets to odd byte addresses, due to the off-by-one-bit error, while all of our instructions are word-aligned.<\/p>\n<p>On the upshot, I was able to hack around this in the assembler source and get a working version.  Plug in the new binary, fix a minor bug in the parameterization of our sign extender and voila:<\/p>\n<p><a href=\"http:\/\/www.mccambridge.org\/blogpics\/wiscfl06.png\"><img src=\"http:\/\/www.mccambridge.org\/blogpics\/wiscfl06_t.png\"\/><\/a><\/p>\n<p>This is the (admittedly meaningless out of context) output of running the <a href=\"http:\/\/www.cs.wisc.edu\/~david\/courses\/cs552\/Project\/testprograms\/branchTest\">branchTest test program<\/a> (assembled by my modified assembler).<\/p>\n<p>To see the full output and verify perfect branching functionality for yourself, here is a link to the <a href=\"http:\/\/www.mccambridge.org\/blogpics\/wiscfl06.pdf\">full waveform [PDF]<\/a>.<\/code><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Dave and I spent all of one late night implementing the first generation (unpipelined) of our 552 project, a 16-bit MIPS processor in Verilog, and then we spent a couple hours this afternoon debugging it. Around 5:15 we ran into a snag: The assembler we were provided was for a slightly different instruction set architecture [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[1],"tags":[],"_links":{"self":[{"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/posts\/141"}],"collection":[{"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/comments?post=141"}],"version-history":[{"count":1,"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/posts\/141\/revisions"}],"predecessor-version":[{"id":1754,"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/posts\/141\/revisions\/1754"}],"wp:attachment":[{"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/media?parent=141"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/categories?post=141"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.mccambridge.org\/blog\/wp-json\/wp\/v2\/tags?post=141"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}