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just a quick question while reading: http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Mips/format.html

Under I-type Instruction, it says:

"In this case, $rt is the destination register, and $rs is the only source register. It is unusual that $rd is not used, and that $rd does not appear in bit positions B25-21 for both R-type and I-type instructions. Presumably, the designers of the MIPS ISA had their reasons for not making the destination register at a particular location for R-type and I-type. "

Can someone explain what the MIPS ISA designers' reasons are?

user3754212
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  • Note that in this link R-type instruction text description doesn't match the R-type figure - looks like there are some mistakes here, I'd look for another MIPS reference. – Sean Barbeau Jun 25 '14 at 01:00
  • I thought that's how it supposed to look. Even my class is teaching in that way. Notice the quote, "designers of the MIPS ISA had their reasons for not making the destination register at a particular location for R-type and I-type." – user3754212 Jun 25 '14 at 18:18
  • I wouldn't pay that much attention to quotes like that, unless you know who wrote that and have reason to believe that he or she wasn't just expressing personal opinions about the ISA. I could just as easily offer the counter-opinion that not renaming bits 16..20 from `rt` to `rd` for I-type instructions was sensible even though `rt` is the destination register for some of those instructions. – Michael Jun 25 '14 at 21:09

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