Appendix B

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 architecture Operands implicitly on top of
  a stack. (Early machines, Intel floating-point.)
• Accumulator architecture One operand is
  implicitly an accumulator (a special register).
  (Early machs.)
• 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 CAB
4
Number of Operands

• 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 bc)
• 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

5
Endians Alignment
Increasing byteaddress
0
1
2
3
4
5
6
7
4
Word-aligned word at byte address 4.
2
Halfword-aligned word at byte address 2.
1
Byte-aligned (non-aligned) word, at byte address
1.
word
Little-endian byte order (least-significant byte
first).
0 (LSB)
1
2
3 (MSB)
word
Big-endian byte order (most-significant byte
first).
3 (MSB)
2
1
0 (LSB)
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
Sign bit is not counted
SPEC CPU2000 on Alpha
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
• 32 64-bit GPRs, R0-R31.
• Really, only 31 R0 is just a constant 0.
• 32 64-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.23-26 in textbook
• Branch and Jump Addresses
• PC-relative addressing
• Branch target address (PC4) (Displacement
  00) Note instead of PC, (PC4) 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