IGCSE Computer Science Paper 1 Definitions Worksheet: 1.1 Number Systems and 1.2 Text, Sound and Images (Modules 28–41)
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Key terms
Write what each term means, in your own words.
1
two's complement
2
-128 64 32 16 8 4 2 1
3
-128 to 127
4
-128, 64, 32, 16, 8, 4, 2, 1
5
positive two's complement
6
flip the bits then add 1
7
character set
8
ASCII
9
8-bit ASCII encoding
Answer Key
IGCSE Computer Science Paper 1 · Modules 28–41 · Student wording will vary for definitions
1two's complement — A way to store negative integers in binary. The leftmost bit shows the sign: 0 means positive, 1 means negative. Positive numbers look the same as ordinary binary.
2-128 64 32 16 8 4 2 1 — The 8-bit two's complement place values. The leftmost bit (the sign bit) is worth -128 when it is 1, and the other seven bits keep their usual values, so 10000000 = -128.
3-128 to 127 — The range of an 8-bit two's complement integer: 10000000 is -128, 11111111 is -1 and 01111111 is 127. Unsigned 8-bit binary gives 0 to 255 instead.
4-128, 64, 32, 16, 8, 4, 2, 1 — Add the place values with a 1 under them to get the denary value. A leftmost 1 means -128 is added: 11110110 = -128+64+32+16+4+2 = -10. A leftmost 0 means positive.
5positive two's complement — A denary number from 0 to 127 is written as ordinary binary, padded with zeros to 8 bits so the leftmost sign bit is 0. Numbers above 127 cannot be stored.
6flip the bits then add 1 — Write the positive value in 8-bit binary, flip every bit, then add 1. Example: 5 is 00000101, flip to 11111010, add 1 to get 11111011 (-5).
7character set — The agreed list of characters a system can use, with a unique binary code for each one, so every device reads the same code as the same character.
8ASCII — A character set that gives each character a 7-bit code (128 characters), usually stored in 8 bits. Extended ASCII uses 8 bits for 256 characters.
98-bit ASCII encoding — Look up each letter's ASCII code and write it as an 8-bit binary group, then join the groups in order. Example: A = 65 = 01000001.