Reading Assignment and Discussion Topics
Computer Science 471

for class on Tuesday Sept. 24, 1996

Please read Sections 3.5 through 3.15 of the Patterson & Hennessy text, take a look at section A.10 in Appendix A, and be prepared to discuss the following issue:

Pages A-58 through A-62 define the branch instructions used in the MIPS assembler. Some of these are pseudoinstructions that stand for one or more actual instructions. You can tell the real instructions from the pseudoinstructions because the latter have no encodings specified. Also, the table of opcodes in Figure A.18 includes the real instructions only.

Thus the real MIPS conditional branch instructions do not include all possible conditions. For some (simple) conditions you need to use more than one instruction: a<b, for example, requires a set-less-than instruction (slt) followed by a branch. Note that a full set of comparisons with zero is included. Note also that the condition being tested always involves reading at least one register.

The SPARC architecture (which many of you studied in COS 217) has a different approach to conditional branches. Here's where to find the 217 lecture slides about this: http://www.cs.princeton.edu/courses/cs471/sparc-branches.ps (it's a Postscript file). The architecture includes four condition codes, which are set by variants of the normal arithmetic and logical instructions: addcc and xorcc, for example, are condition-code-setting variants of the add and xor instructions. (Here is roughly what the condition codes mean: N means negative, Z means zero, C means carry out, and V means overflow.) The conditional branch instructions test the condition codes only, and not the contents of the registers.

How would you compare the MIPS and SPARC schemes?