MIPS Assembly Language Programming

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1.3. REPRESENTING PROGRAMS 11 Figure 1.5: The ASCII Character Set 00 NUL 01 SOH 02 STX 03 ETX 04 EOT 05 ENQ 06 ACK 07 BEL 08 BS 09 HT 0A NL 0B VT 0C NP 0D CR 0E SO 0F SI 10 DLE 11 DC1 12 DC2 13 DC3 14 DC4 15 NAK 16 SYN 17 ETB 18 CAN 19 EM 1A SUB 1B ESC 1C FS 1D GS 1E RS 1F US 20 SP 21 ! 22 " 23 # 24 $ 25 % 26 & 27 ’ 28 ( 29 ) 2A * 2B + 2C , 2D - 2E . 2F / 30 0 31 1 32 2 33 3 34 4 35 5 36 6 37 7 38 8 39 9 3A : 3B ; 3C < 3D = 3E > 3F ? 40 @ 41 A 42 B 43 C 44 D 45 E 46 F 47 G 48 H 49 I 4A J 4B K 4C L 4D M 4E N 4F O 50 P 51 Q 52 R 53 S 54 T 55 U 56 V 57 W 58 X 59 Y 5A Z 5B [ 5C 5D ] 5E ^ 5F 60 ` 61 a 62 b 63 c 64 d 65 e 66 f 67 g 68 h 69 i 6A j 6B k 6C l 6D m 6E n 6F o 70 p 71 q 72 r 73 s 74 t 75 u 76 v 77 w 78 x 79 y 7A z 7B { 7C | 7D } 7E ~ 7F DEL or Japanese). Already new character sets which address these problems (and can be used to represent characters of many languages side by side) are being proposed, and eventually there will unquestionably be a shift away from ASCII to a new multilanguage standard 1 . 1.3 Representing Programs Just as groups of bits can be used to represent numbers, they can also be used to represent instructions for a computer to perform. Unlike the two’s complement notation for integers, which is a standard representation used by nearly all computers, the representation of instructions, and even the set of instructions, varies widely from one type of computer to another. The MIPS architecture, which is the focus of later chapters in this document, uses 1 This shift will break many, many existing programs. Converting all of these programs will keep many, many programmers busy for some time.
12 CHAPTER 1. DATA REPRESENTATION a relatively simple and straightforward representation. Each instruction is exactly 32 bits in length, and consists of several bit fields, as depicted in figure 1.6. Figure 1.6: MIPS R2000 Instruction Formats 6 bits 5 bits 5 bits 5 bits 5 bits 6 bits Register op reg1 reg2 des shift funct Immediate op reg1 reg2 16-bit constant Jump op 26-bit constant The first six bits (reading from the left, or high-order bits) of each instruction are called the op field. The op field determines whether the instruction is a register, immediate, or jump instruction, and how the rest of the instruction should be interpreted. Depending on what the op is, parts of the rest of the instruction may represent the names of registers, constant memory addresses, 16-bit integers, or other additional qualifiers for the op. If the op field is 0, then the instruction is a register instruction, which generally perform an arithmetic or logical operations. The funct field specifies the operation to perform, while the reg1 and reg2 represent the registers to use as operands, and the des field represents the register in which to store the result. For example, the 32-bit hexadecimal number 0x02918020 represents, in the MIPS instruction set, the operation of adding the contents of registers 20 and 17 and placing the result in register 16. Field op reg1 reg2 des shift funct Width 6 bits 5 bits 5 bits 5 bits 5 bits 6 bits Values 0 20 17 16 0 add Binary 000000 10100 10001 10000 00000 100000 If the op field is not 0, then the instruction may be either an immediate or jump instruction, depending on the value of the op field. 1.4 Memory Organization We’ve seen how sequences of binary digits can be used to represent numbers, characters, and instructions. In a computer, these binary digits are organized and ma-
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