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number systems

summary

    This subchapter discusses number systems.

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number systems

    This subchapter discusses number systems.

assembly language instructions

number systems

    Binary is a number system using only ones and zeros (or two states).

    Decimal is a number system based on ten digits (including zero).

    Hexadecimal is a number system based on sixteen digits (including zero).

    Octal is a number system based on eight digits (including zero).

    Duodecimal is a number system based on twelve digits (including zero).

binary	octal	decimal	duodecimal	hexadecimal
0	0	0	0	0
1	1	1	1	1
10	2	2	2	2
11	3	3	3	3
100	4	4	4	4
101	5	5	5	5
110	6	6	6	6
111	7	7	7	7
1000	10	8	8	8
1001	11	9	9	9
1010	12	10	A	A
1011	13	11	B	B
1100	14	12	10	C
1101	15	13	11	D
1110	16	14	12	E
1111	17	15	13	F
10000	20	16	14	10
10001	21	17	15	11
10010	22	18	16	12
10011	23	19	17	13
10100	24	20	18	14
10101	25	21	19	15
10110	26	22	1A	16
10111	27	23	1B	17
11000	30	24	20	18

integer representations

    Sign-magnitude is the simplest method for representing signed binary numbers. One bit (by universal convention, the highest order or leftmost bit) is the sign bit, indicating positive or negative, and the remaining bits are the absolute value of the binary integer. Sign-magnitude is simple for representing binary numbers, but has the drawbacks of two different zeros and much more complicates (and therefore, slower) hardware for performing addition, subtraction, and any binary integer operations other than complement (which only requires a sign bit change).

    In one’s complement representation, positive numbers are represented in the “normal” manner (same as unsigned integers with a zero sign bit), while negative numbers are represented by complementing all of the bits of the absolute value of the number. Numbers are negated by complementing all bits. Addition of two integers is peformed by treating the numbers as unsigned integers (ignoring sign bit), with a carry out of the leftmost bit position being added to the least significant bit (technically, the carry bit is always added to the least significant bit, but when it is zero, the add has no effect). The ripple effect of adding the carry bit can almost double the time to do an addition. And there are still two zeros, a positive zero (all zero bits) and a negative zero (all one bits).

    In two’s complement representation, positive numbers are represented in the “normal” manner (same as unsigned integers with a zero sign bit), while negative numbers are represented by complementing all of the bits of the absolute value of the number and adding one. Negation of a negative number in two’s complement representation is accomplished by complementing all of the bits and adding one. Addition is performed by adding the two numbers as unsigned integers and ignoring the carry. Two’s complement has the further advantage that there is only one zero (all zero bits). Two’s complement representation does result in one more negative number (all one bits) than positive numbers.

    Two’s complement is used in just about every binary computer ever made. Most processors have one more negative number than positive numbers. Some processors use the “extra” neagtive number (all one bits) as a special indicator, depicting invalid results, not a number (NaN), or other special codes.

    In unsigned representation, only positive numbers are represented. Instead of the high order bit being interpretted as the sign of the integer, the high order bit is part of the number. An unsigned number has one power of two greater range than a signed number (any representation) of the same number of bits.

bit pattern	sign-mag.	one’s comp.	two’s comp	unsigned
000	0	0	0	0
001	1	1	1	1
010	2	2	2	2
011	3	3	3	3
100	-0	-3	-4	4
101	-1	-2	-3	5
110	-2	-1	-2	6
111	-3	-0	-1	7

floating point representations

    Floating point numbers are the computer equivalent of “scientific notation” or “engineering notation”. A floating point number consists of a fraction (binary or decimal) and an exponent (bianry or decimal). Both the fraction and the exponent each have a sign (positive or negative).

    In the past, processors tended to have proprietary floating point formats, although with the development of an IEEE standard, most modern processors use the same format. Floating point numbers are almost always binary representations, although a few early processors had (binary coded) decimal representations. Many processors (especially early mainframes and early microprocessors) did not have any hardware support for floating point numbers. Even when commonly available, it was often in an optional processing unit (such as in the IBM 360/370 series) or coprocessor (such as in the Motorola 680x0 and pre-Pentium Intel 80x86 series).

    Hardware floating point support usually consists of two sizes, called single precision (for the smaller) and double precision (for the larger). Usually the double precision format had twice as many bits as the single precision format (hence, the names single and double). Double precision floating point format offers greater range and precision, while single precision floating point format offers better space compaction and faster processing.

    F_floating format (single precision floating), DEC VAX, 32 bits, the first bit (high order bit in a register, first bit in memory) is the sign magnitude bit (one=negative, zero=positive or zero), followed by 15 bits of an excess 128 binary exponent, followed by a normalized 24-bit fraction with the redundant most significant fraction bit not represented. Zero is represented by all bits being zero (allowing the use of a longword CLR to set a F_floating number to zero). Exponent values of 1 through 255 indicate true binary exponents of -127 through 127. An exponent value of zero together with a sign of zero indicate a zero value. An exponent value of zero together with a sign bit of one is taken as reserved (which produces a reserved operand fault if used as an operand for a floating point instruction). The magnitude is an approximate range of .29*10^-38 through 1.7*10³⁸. The precision of an F_floating datum is approximately one part in 2²³, or approximately seven (7) decimal digits).

    32 bit floating format (single precision floating), AT&T DSP32C, 32 bits, the first bit (high order bit in a register, first bit in memory) is the sign magnitude bit (one=negative, zero=positive or zero), followed by 23 bits of a normalized two’s complement fractional part of the mantissa, followed by an eight bit exponent. The magnitude of the mantissa is always normalized to lie between 1 and 2. The floating point value with exponent equal to zero is reserved to represent the number zero (the sign and mantissa bits must also be zero; a zero exponent with a nonzero sign and/or mantissa is called a “dirty zero” and is never generated by hardware; if a dirty zero is an operand, it is treated as a zero). The range of nonzero positive floating point numbers is N = [1 * 2^-127, [2-2^-23] * 2¹²⁷] inclusive. The range of nonzero negative floating point numbers is N = [-[1 + 2^-23] * 2^-127, -2 * 2¹²⁷] inclusive.

    40 bit floating format (extended single precision floating), AT&T DSP32C, 40 bits, the first bit (high order bit in a register, first bit in memory) is the sign magnitude bit (one=negative, zero=positive or zero), followed by 31 bits of a normalized two’s complement fractional part of the mantissa, followed by an eight bit exponent. This is an internal format used by the floating point adder, accumulators, and certain DAU units. This format includes an additional eight guard bits to increase accuracy of intermediate results.

    D_floating format (double precision floating), DEC VAX, 64 bits, the first bit (high order bit in a register, first bit in memory) is the sign magnitude bit (one=negative, zero=positive or zero), followed by 15 bits of an excess 128 binary exponent, followed by a normalized 48-bit fraction with the redundant most significant fraction bit not represented. Zero is represented by all bits being zero (allowing the use of a quadword CLR to set a D_floating number to zero). Exponent values of 1 through 255 indicate true binary exponents of -127 through 127. An exponent value of zero together with a sign of zero indicate a zero value. An exponent value of zero together with a sign bit of one is taken as reserved (which produces a reserved operand fault if used as an operand for a floating point instruction). The magnitude is an approximate range of .29*10^-38 through 1.7*10³⁸. The precision of an D_floating datum is approximately one part in 2⁵⁵, or approximately 16 decimal digits).

See also Data Representation in Assembly Language

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