Arithmetic and Logic Instructions

A Course in Microprocessor

Electrical Engineering Department

University of Indonesia

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Addition-Subtraction-Comparison

Whenever arithmetic and logic instruction execute, the contents of the flag register change

AdditionTable 5.1 illustrates the addressing modes available to the ADD instructions –Register Addition

• Example 5.1 show a sample of register addition

–Immediate Addition• Example 5.2 shows 8-bit immediate addition example

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Addition-Subtraction-Comparison (cont’d)

– Memory-to-Register Addition• Example 5.3 adds two consecutive bytes of data

– Array Addition• Example 5.4 shows a procedure that adds the contents of array

elements 3, 5, and 7• Example 5.5 shows the scaled-index form of addressing to add

elements 3, 5, and 7 of an area of memory called ARRAY

– Increment Addition• Examples 5.6 modifies example 5.3 to use the increment instruction

for addressing NUMB and NUMB+1 (see also Table 5.2)

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Addition-Subtraction-Comparison (cont’d)

– Addition-with-Carry• Table 5.3 lists several add-with-carry instruction

• Figure 5.1 illustrates the addition

• Ex. 5.7 and Ex. 5.8 show the short program

Subtraction Table 5.4 shows addressing modes for the subtraction

instruction

– Register Subtraction• Example 5.9

– Immediate Subtraction• Example 5.10

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Addition-Subtraction-Comparison (cont’d)

– Decrement Subtraction• It subtracts a 1 from a register or the contents of a memory

location (see Table 5.5)

– Subtract-with-Borrow• It functions as a regular subtraction, except that the carry flag (C)

which holds the borrow, also subtracts from the difference (see Table 5.6 & Ex. 5.11)

Comparison– The comparison instruction (CMP) is a subtraction that

changes only the flag bits

– See Table 5.7 and Example 5.12

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Multiplication and Division Only modern p contain multiplication and division instructions

Multiplication multiplication (bytes, words, or double-words) can be signed

integer (IMUL) or unsigned (MUL)

– 8-bit Multiplication• The multiplicand is always in the register AL

• See Table 5.8 and Example 5.13

– 16-bit Multiplication• AX contains the multiplicand and the product appears in

DX-AX

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Multiplication and Division (cont’d)

– 32-bit Muliplication• The contents of EAX are multiplied by the operand specified with

the instruction

• The product (64-bits wide) is found in EDX-EAX where EAX contains the LS32B (see Table 5.10)

Division None of the flag bits change predictably; i.e., a division can result is

two different types of error:

– an attempt to divide by zero

– a divide overflow (see the 3rd paragraph in p.157)

– In both cases, the p generates an interrupt if a divide error occurs

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Multiplication and Division (cont’d)

– 8-bit Division• The AX register stores the dividend

• After the division, AL contains the quotient and AH contains a whole number remainder

• See Table 5.11, Example 5.14 & Example 5.15

– 16-bit Division• Instead of dividing into AX, the 16-bit number is divided

into DX-AX, a 32-bit dividend

• The quotient appears in AX and the remainder in DX after a 16-bit division

• See Table 5.12 and Example 5.16

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Multiplication and Division (cont’d)

– 32-bit Division• The 64-bit contents of EDX-EAX are divided by the

operand specified by the instruction, leaving a 32-bit quotient in EAX and a 32-bit remainder in EDX

• See Table 5.13

– The Remainder• After a division, the remainder could be use to

round the result or dropped to truncate the result or conver-ted to a fractional remainder

• Study Example 5.17 and Example 5.18

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BCD and ASCII Arithmetic

BCD Arithmetic p allows arithmetic manipulation of both BCD and ASCII

– DAA (Decimal Adjust After Addition) Instruction• It follows the ADD or ADC instruction to adjust the result

into a BCD result (Ex. 5.19)

– DAS (Decimal Adjust After Subtraction) Instruction• It functions as does the DAA, except that it follows a

subtraction instead of an addition (Ex. 5.20)

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BCD and ASCII Arithmetic (cont’d)

ASCII Arithmetic– AAA (ASCII Adjust After Addition)

• Example 5.21

– AAD (ASCII Adjust Before Division)• Example 5.22

– AAM (ASCII Adjust After Multiplication)• Example 5.23, Example 5.24, Example 5.25

– AAS (ASCII Adjust After Subtraction)• Adjust the AX register after an ASCII subtraction

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Basic Logic Instruction Logic operations provide binary bit control in low-level software;

allow bits to be set, cleared, or complemented

AND• Performs logical multiplication as depicted by the truth table in

Fig.5.3 and Fig. 5.4• See also Ex. 5.26 and Table 5.14

OR• Performs logical addition as depicted in Fig. 5.5 and Ex. 5.27

and Fig. 5.6 and Table 5.15

X-OR• Study Fig. 5.7, Table 5.16 and Ex 5.28

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Basic Logic Instruction (cont’d)

Test and Bit Test Instruction• Test instruction performs the AND operation; the

difference is that the AND instruction changes the destination operand, while the TEST does not

• Test instruction affects only the flag (Table 5.17 and Example 5.29)

• Bit Test instruction tests single bit position (Table 5.18 and Example 5.30

NOT and NEG• NOT performs logical inversion (1’s complement) and

NEG performs arithmetic sign inversion (2’s complement)

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Shift and Rotate

Shift and Rotate instructions manipulate binary numbers at the binary bit level

Shifts and Rotates find their most common application in low-level software used to control I/O devices

Shifts• Shifts position or move numbers to the left or right within

a register or memory location• Shifts also perform simple arithmetic such as

multiplication by powers of 2+n (left shift) and division by powers of 2-n (right shift)

• Study fig. 5.9, Table 5.20, Examples 5.31 & 5.32

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Shift and Rotate (cont’d)

Rotate• Rotates position binary data by rotating the infromation in

a register or memory location either from one end to another or through the carry flag

• Rotates are often used to shift wide numbers to the left or right

• Study Fig. 5.10, Table 5.21, Example 5.33

Bit Scan Instructions• BSF (bit scan forward) and BSR (bit scan reverse) scan

through a number searching for the first 1-bit encountered

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String Comparisons It is very powerful because allows to manipulate large blocks of data

with relative ease

SCAS– SCAS compares the AL register with a byte block of memory

(SCASB), the AX register with a word block of memory (SCASW), or the EAX register with a doubleword block of memory (SCASD)

– study Example 5.34 and Example 5.35

CMPS– It always compares two sections of memory data as bytes (CMPSB),

word (CMPSW), or doubleword (CMPSD); Study Example 5.36

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