Instruction Set Architecture (ISA) Data Types

Transcription

1 Instruction Set Architecture (ISA) Data Types Computer Systems: Section 4.1

2 Suppose you built a computer What Building Blocks would you use?

3 Arithmetic Logic Unit (ALU) OP1 OP2 OPERATION ALU RES

4 Full Adder SUM = A B C CarryOut = A B + A CarryIn + B CarryIn

5 Shifters

6 Hardware (Gate) Implementation 8 bit bitwise AND

7 Arithmetic Logic Unit (ALU) OP2 OP1 ALU BITWISE AND OPCODE RES

8 Computer Architecture Wikipedia a set of rules and methods that describe the functionality, organization, and implementation of computer systems the capabilities and programming model of a computer but not a particular implementation We will study the x86 architecture. Part of the architecture defines what type of data the hardware can operate on.

9 X86 Data Types Byte - 8 bit binary or ASCII character Word - 16 bit binary Double Word - 32 bit binary Quad Word - 64 bit binary 32, 64, or 128 bit floating point

10 ALU + Registers R0: 0x F0 R1: 0x F1 R2: 0xFFFF FFFF R3: OPERATION ALU

11 X86 Integer Registers bit Registers Fast Read/Write (CPU Speed) No explicit data type! Values undefined (X) until set Register Names based on Historic Usage Naming conventions used to access portions of registers

12 x86 Integer Registers Origin %rax %eax %ax %ah %al Accumulate %rcx %ecx %cx %ch %cl Counter %rdx %edx %dx %dh %dl Data %rbx %ebx %bx %bh %bl Base %rsi %esi %si Source Index %rdi %edi %di Destination Index %rsp %esp %sp Stack Pointer %rbp %ebp %bp base Pointer

13 ALU + Registers + Memory R0: 0x F0 R1: 0x F1 R2: 0xFFFF FFFF R3: 0x F0 0x F1 0xFFFF FFFF OP ALU

14 Computer vs. Adding Machine What is the difference?

15 ALU+Registers+Memory+Instructions R0: 0x F0 R1: 0x F1 R2: 0xFFFF FFFF R3: 0x F0 0x F1 0xFFFF FFFF ALU CONTROL UNIT

16 ISA Contents The data types the instructions can work on two s complement binary, ascii character, unsigned binary, etc. The instructions the hardware recognizes add, move, get, The data the instructions can work on Registers Memory The external interfaces supported by the instructions File I/O Exception Handling and Interrupts Software ISA

17 Abstraction Bits are stored in memory from most significant at left to least significant at right (int 100,000 = 0x A0) A 0 Byte m Byte m+1 Byte m+2 Byte m+3

18 Leaky Abstraction Endian -ness Some machines store as expected (Big endian) S b 31 b 30 b 29 b 28 b 27 b 26 b 25 b 24 b 23 b 22 b 21 b 20 b 19 b 18 b 17 b 16 b 15 b 14 b 13 b 12 b 11 b 10 b 9 b 8 b 7 b 6 b 5 b 4 b 3 b 2 b 1 b A 0 Byte 42 Byte 43 Byte 44 Byte 45 Some machines store least significant byte first! (Little-endian) S b 31 b 30 b 29 b 28 b 27 b 26 b 25 b 24 b 23 b 22 b 21 b 20 b 19 b 18 b 17 b 16 b 15 b 14 b 13 b 12 b 11 b 10 b 9 b 8 b 7 b 6 b 5 b 4 b 3 b 2 b 1 b A Byte 42 Byte 43 Byte 44 Byte 45

19 Why Little Endian? Casting: int x; /* 32 bits starting at byte 42 */ y = (short int) x; /* Put the least significant 16 bits from x into y */ Big Endian A x (short int) x Little Endian A x (short int) x

20 Full Adder SUM = A B CarryIn CarryOut = A&B A&CarryIn B&CarryIn

21 Eight Bit Adder CI OP1 RESULT OP2 CO

22 Big Endian vs. Little Endian Adder

23 When does Endian-ness Leak? Only on multi-byte data types (short, int, long, float, double) Only when we look at the underlying bits Big-endian machine: First byte is the most significant byte Everything works until we get binary data from a little-endian machine Little-endian machine: First byte is the least significant byte When hardware interprets the bits/bytes as a number, bytes are switched We don t even know if a machine is big-endian or little-endian! Until: we get binary data from a big-endian machine OR Until we look at the bit representation of the data, not treated as a number

24 Managing Endian-Ness Network standard is big-endian stdlib functions machine representation network (big-endian) representation htons (short), htonl (long) Network representation (big-endian) machine representation ntohs (short), ntohl (long) No-ops when hardware is big-endian endian.h functions htobe16, htobe32, htobe64, htole16, htole32, htole64 be16toh, be32toh, be64toh, le16toh, le32toh, le64toh See xmp_endian/network.c

25 Endian-ness in this Course LDAP machines are little-endian When you put binary data in a file, you must enter numbers in little-endian format! When you print memory from gdb (use the x examine command), if you specify byte output formats, data will appear little endian. (When you specify multi-byte numeric output formats, the hardware switches the bytes for you.) If you make a union of byte-data and multi-byte formats, the byte data will show endian-ness Otherwise, everything looks like big-endian!