1 Appendix B Classifying Instruction Set Architecture Memory addressing mode Operations in the instruction set Control flow instructions Instruction format CDA5155 Spring, 2007, Peir / University of Florida

2 Classifying Architectures One important classification scheme is by the type of addressing modes supported. –Stack –Stack architecture: Operands implicitly on top of a stack. (Early machines, Intel floating-point.) –Accumulator –Accumulator architecture: One operand is implicitly an accumulator (a special register). (Early machs.) –General-purpose register –General-purpose register architecture: Operands may be any of a large (typically 10s-100s) # of registers. Register-memory architectures: One op may be memory. Load-store architectures: All ops are registers, except in special load and store instructions.

3 Four Architecture Classes Assembly for C:=A+B :

4 A further classification is by the maximum number of operands, and # that can be memory: e.g., –2-operand (e.g. a += b) src/dest(reg), src(reg) src/dest(reg), src(mem)IBM 360, x86, 68k src/dest(mem), src(mem) VAX –3-operand (e.g. a = b+c) dest(reg), src1(reg), src2(reg) MIPS, PPC, SPARC, &c. dest(reg), src1(reg), src2(mem) IBM 370 dest(mem), src1(mem), src2(mem)IBM 370, VAX Number of Operands

5 Endians & Alignment Word-aligned word at byte address 4. Byte-aligned (non-aligned) word, at byte address 1. 2 Halfword-aligned word at byte address 2. Increasing byte address 0 (LSB)123 (MSB) 210 (LSB) Little-endian byte order (least-significant byte “first”). Big-endian byte order (most-significant byte “first”). word

6 Addressing Modes In example assembly syntax in middle column, ( ) indicates memory access. (A typical syntax.) In RTL syntax on right, [ ] denotes accessing a member of an array, Register or Memory.

7 Addressing Mode Usage 3 SPEC89 on VAX

8 Displacement Distribution SPEC CPU2000 on Alpha Sign bit is not counted

9 Use of Immediate Operand SPEC CPU2000 on Alpha

10 Distribution of Immediate SPEC CPU2000 on Alpha Sign bit is not counted

11 Instruction Type (Same as B.12 in the 4th Edition)

12 Instruction Distribution (Same as Fig. 2.16) (5 SPECint92)

13 Control Flow Instructions Four basic types: –(Conditional) branches –(Unconditional) jumps –Procedure calls –Procedure returns Control flow addressing modes: –Often PC-relative (PC + displacement). Relocatable. –Also useful: register indirect jumps (reg. has addr.). Uses: Procedure returns Case / switch statements Virtual functions / methods (abstract class method calls) High-order functions / function pointers Dynamically shared libraries

14 Conditional Branch Options Condition Code (CC) Register –E.g.: X86, ARM, PPC, SPARC, … –ALU ops set condition code flags in the CCR –Branch just checks the flag Condition register –E.g.: Alpha, MIPS –Comparison instruction puts result in a GPR –Branch instruction checks the register Compare & Branch –E.g.: PA-RISC, VAX –Compare & branch in 1 instruction.

15 Procedure Calling Conventions Two major calling conventions: –Caller saves: Before the call, procedure caller saves registers that will be needed later, even if callee did not use them –Callee saves: Inside the call, called procedure saves registers that it will overwrite Can be more efficient if many small procedures Many architectures use a combination of schemes: –E.g., MIPS: Some registers caller-saves, some callee- saves

16 Three Classes of Control Instructions SPEC CPU2000 on Alpha

17 Branch Distance Distribution SPEC CPU2000 on Alpha

18 Branch Comparison Types SPEC CPU2000 on Alpha

19 Encoding An Instruction Set

20 MIPS Architecture RISC, load-store architecture, simple address 32-bit instructions, fixed format bit GPRs, R0-R31. –Really, only 31 – R0 is just a constant bit FPRs, F0-F31 –Can hold 32-bit floats also (with other ½ unused). –“SIMD” extensions operate on more floats in 1 FPR A few special registers –Floating-point status register Load/store 8-, 16-, 32-, 64-bit integers –All sign-extended to fill 64-bit GPR –Also 32- bit floats/doubles

21 MIPS Addressing Modes Register (arith./logical ops only) Immediate (arith./logical only) & Displacement (load/stores only) –16-bit immediate / offset field –Register indirect: use 0 as displacement offset –Direct (absolute): use R0 as displacement base Byte-addressed memory, 64-bit address Software-settable big-endian/little-endian flag Alignment required

22 Inst. Format: I-type Instructions

23 Inst. Format: R-type Instructions

24 Inst. Format: J-type Instructions

25 MIPS Instruction Set Go through Figures B in textbook Branch and Jump Addresses –PC-relative addressing: (PC+4) wordBranch target address = (PC+4) + (Displacement || “00”); Note instead of PC, (PC+4) for hardware convenience and the two zeros is added due to word displacement. –Jump (limited to 256MB range): (Upper 4 bits of Current PC) || (26-bits address in Jump) || (“00”) –Long Jump: Jump register: save 32-bit target address in register