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In computer architecture, 8-bit integers, memory addresses, or other data units are those that are 8 bits (1 octet) wide. Also, 8-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 8-bit is also a term given to a generation of microcomputers in which 8-bit microprocessors were the norm.

1 2 4 8 12 16 18 24 26 31 32 36 48 60 64 128 256 512
16 32 64
×½ ×1 ×2 ×4 ×8
32 64 128

The IBM System/360 introduced byte-addressable memory with 8-bit bytes, as opposed to bit-addressable or decimal digit-addressable or word-addressable memory, although its general purpose registers were 32 bits wide, and addresses were contained in the lower 24 bits of those addresses. Different models of System/360 had different internal data path widths; the IBM System/360 Model 30 (1965) implemented the 32-bit System/360 architecture, but had an 8 bit native path width, and performed 32-bit arithmetic 8 bits at a time.[1]

The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system; it had 8-bit data words and 16-bit addresses. The Zilog Z80 (compatible with the 8080) and the Motorola 6800 were also used in similar computers. The Z80 and the MOS Technology 6502 8-bit CPUs were widely used in home computers and second- and third-generation game consoles of the 1970s and 1980s. Many 8-bit CPUs or microcontrollers are the basis of today's ubiquitous embedded systems.

Contents

DetailsEdit

There are 28 (256) different possible values for 8 bits. When unsigned, it has possible values ranging from 0 to 255, when signed, it has -128 to 127.

Eight-bit CPUs use an 8-bit data bus and can therefore access 8 bits of data in a single machine instruction. The address bus is typically a double octet wide (i.e. 16-bit), due to practical and economical considerations. This implies a direct address space of only 64 KB on most 8-bit processors.

Binary Octal Decimal Hexadecimal
00000000 0 0 0
00000001 1 1 1
00000010 2 2 2
00000011 3 3 3
00000100 4 4 4
00000101 5 5 5
00000110 6 6 6
00000111 7 7 7
00001000 10 8 8
00001001 11 9 9
00001010 12 10 A
00001011 13 11 B
00001100 14 12 C
00001101 15 13 D
00001110 16 14 E
00001111 17 15 F
00010000 20 16 10
00010001 21 17 11
00010010 22 18 12
00010011 23 19 13
00010100 24 20 14
00010101 25 21 15
00010110 26 22 16
00010111 27 23 17
00011000 30 24 18
00011001 31 25 19
00011010 32 26 1A
00011011 33 27 1B
00011100 34 28 1C
00011101 35 29 1D
00011110 36 30 1E
00011111 37 31 1F
00100000 40 32 20
00100001 41 33 21
00100010 42 34 22
00100011 43 35 23
00100100 44 36 24
00100101 45 37 25
00100110 46 38 26
00100111 47 39 27
00101000 50 40 28
00101001 51 41 29
00101010 52 42 2A
00101011 53 43 2B
00101100 54 44 2C
00101101 55 45 2D
00101110 56 46 2E
00101111 57 47 2F
00110000 60 48 30
00110001 61 49 31
00110010 62 50 32
00110011 63 51 33
00110100 64 52 34
00110101 65 53 35
00110110 66 54 36
00110111 67 55 37
00111000 70 56 38
00111001 71 57 39
00111010 72 58 3A
00111011 73 59 3B
00111100 74 60 3C
00111101 75 61 3D
00111110 76 62 3E
00111111 77 63 3F
01000000 100 64 40
01000001 101 65 41
01000010 102 66 42
01000011 103 67 43
01000100 104 68 44
01000101 105 69 45
01000110 106 70 46
01000111 107 71 47
01001000 110 72 48
01001001 111 73 49
01001010 112 74 4A
01001011 113 75 4B
01001100 114 76 4C
01001101 115 77 4D
01001110 116 78 4E
01001111 117 79 4F
01010000 120 80 50
01010001 121 81 51
01010010 122 82 52
01010011 123 83 53
01010100 124 84 54
01010101 125 85 55
01010110 126 86 56
01010111 127 87 57
01011000 130 88 58
01011001 131 89 59
01011010 132 90 5A
01011011 133 91 5B
01011100 134 92 5C
01011101 135 93 5D
01011110 136 94 5E
01011111 137 95 5F
01100000 140 96 60
01100001 141 97 61
01100010 142 98 62
01100011 143 99 63
01100100 144 100 64
01100101 145 101 65
01100110 146 102 66
01100111 147 103 67
01101000 150 104 68
01101001 151 105 69
01101010 152 106 6A
01101011 153 107 6B
01101100 154 108 6C
01101101 155 109 6D
01101110 156 110 6E
01101111 157 111 6F
01110000 160 112 70
01110001 161 113 71
01110010 162 114 72
01110011 163 115 73
01110100 164 116 74
01110101 165 117 75
01110110 166 118 76
01110111 167 119 77
01111000 170 120 78
01111001 171 121 79
01111010 172 122 7A
01111011 173 123 7B
01111100 174 124 7C
01111101 175 125 7D
01111110 176 126 7E
01111111 177 127 7F
10000000 200 128 80
10000001 201 129 81
10000010 202 130 82
10000011 203 131 83
10000100 204 132 84
10000101 205 133 85
10000110 206 134 86
10000111 207 135 87
10001000 210 136 88
10001001 211 137 89
10001010 212 138 8A
10001011 213 139 8B
10001100 214 140 8C
10001101 215 141 8D
10001110 216 142 8E
10001111 217 143 8F
10010000 220 144 90
10010001 221 145 91
10010010 222 146 92
10010011 223 147 93
10010100 224 148 94
10010101 225 149 95
10010110 226 150 96
10010111 227 151 97
10011000 230 152 98
10011001 231 153 99
10011010 232 154 9A
10011011 233 155 9B
10011100 234 156 9C
10011101 235 157 9D
10011110 236 158 9E
10011111 237 159 9F
10100000 240 160 A0
10100001 241 161 A1
10100010 242 162 A2
10100011 243 163 A3
10100100 244 164 A4
10100101 245 165 A5
10100110 246 166 A6
10100111 247 167 A7
10101000 250 168 A8
10101001 251 169 A9
10101010 252 170 AA
10101011 253 171 AB
10101100 254 172 AC
10101101 255 173 AD
10101110 256 174 AE
10101111 257 175 AF
10110000 260 176 B0
10110001 261 177 B1
10110010 262 178 B2
10110011 263 179 B3
10110100 264 180 B4
10110101 265 181 B5
10110110 266 182 B6
10110111 267 183 B7
10111000 270 184 B8
10111001 271 185 B9
10111010 272 186 BA
10111011 273 187 BB
10111100 274 188 BC
10111101 275 189 BD
10111110 276 190 BE
10111111 277 191 BF
11000000 300 192 C0
11000001 301 193 C1
11000010 302 194 C2
11000011 303 195 C3
11000100 304 196 C4
11000101 305 197 C5
11000110 306 198 C6
11000111 307 199 C7
11001000 310 200 C8
11001001 311 201 C9
11001010 312 202 CA
11001011 313 203 CB
11001100 314 204 CC
11001101 315 205 CD
11001110 316 206 CE
11001111 317 207 CF
11010000 320 208 D0
11010001 321 209 D1
11010010 322 210 D2
11010011 323 211 D3
11010100 324 212 D4
11010101 325 213 D5
11010110 326 214 D6
11010111 327 215 D7
11011000 330 216 D8
11011001 331 217 D9
11011010 332 218 DA
11011011 333 219 DB
11011100 334 220 DC
11011101 335 221 DD
11011110 336 222 DE
11011111 337 223 DF
11100000 340 224 E0
11100001 341 225 E1
11100010 342 226 E2
11100011 343 227 E3
11100100 344 228 E4
11100101 345 229 E5
11100110 346 230 E6
11100111 347 231 E7
11101000 350 232 E8
11101001 351 233 E9
11101010 352 234 EA
11101011 353 235 EB
11101100 354 236 EC
11101101 355 237 ED
11101110 356 238 EE
11101111 357 239 EF
11110000 360 240 F0
11110001 361 241 F1
11110010 362 242 F2
11110011 363 243 F3
11110100 364 244 F4
11110101 365 245 F5
11110110 366 246 F6
11110111 367 247 F7
11111000 370 248 F8
11111001 371 249 F9
11111010 372 250 FA
11111011 373 251 FB
11111100 374 252 FC
11111101 375 253 FD
11111110 376 254 FE
11111111 377 255 FF

Notable 8-bit CPUsEdit

The first commercial 8-bit processor was the Intel 8008 (1972) which was originally intended for the Datapoint 2200 intelligent terminal. Most competitors to Intel started off with such character oriented 8-bit microprocessors. Modernized variants of these 8-bit machines are still one of the most common types of processor in embedded systems.

Another notable 8-bit CPU is the MOS Technology 6502; it, and variants of it, were used in a number of personal computers such as the Apple I and Apple II, the Atari 8-bit family, the BBC Micro, and the Commodore PET and Commodore VIC-20, and in a number of video game consoles such as the Atari 2600 and the Nintendo Entertainment System.

Early or popular 8-bit processors (incomplete)
Manufacturer Processor Year Comment
Intel 8008 1972 Datapoint 2200 compatible
Signetics 2650 1973
Intel 8080 1974 8008 source compatible
Motorola 6800 1974
Fairchild F8 1975
MOS 6502 1975 Similar to 6800, but incompatible
Microchip PIC 1975 Harvard architecture microcontroller
RCA 1802 1976
Zilog Z80 1976 8080 binary compatible
Intel 8085 1977 8080 binary compatible
Motorola 6809 1978 6800 source compatible
Zilog Z8 1978 Harvard architecture microcontroller
Intel 8051 1980 Harvard architecture microcontroller
MOS 6510 1982 Enhanced 6502 custom-made for use in the Commodore 64
Ricoh 2A03 1982 6502 clone minus BCD instructions for the Nintendo Entertainment System
Zilog Z180 1985 Z80 binary compatible
Motorola 68HC11 1985
Atmel AVR 1996
Zilog EZ80 1999 Z80 binary compatible
Infineon XC800 2005
Freescale 68HC08
Hudson HuC6280
Motorola 6803
NEC 78K0[2]

See alsoEdit

  • Vasm, a free assembler

ReferencesEdit

  1. ^ Amdahl, G. M.; Blaauw, G. A.; Brooks, F. P. (1964). "Architecture of the IBM System/360". IBM Journal of Research and Development. 8 (2): 87–101. doi:10.1147/rd.82.0087. 
  2. ^ NEC 78K0