Arithmetic and Logic

Arithmetic Instructions - Flags are set.

The Commands

Arithmetic	Logic	Bitwise
Add - Addition	AND - Logical AND - 1 AND 1 gives 1. Any other input gives 0.	ROL - Rotate bits left. Bit at left end moved to right end.
Sub - Subtraction	OR - Logical OR - 0 OR 0 gives 0. Any other input gives 1.	ROR - Rotate bits right. Bit at right end moved to left end.
Mul - Multiplication	XOR - Logical exclusive OR - Equal inputs give 0. Non equal inputs give 1.	SHL - Shift bits left and discard leftmost bit.
Div - Division	NOT - Logical NOT - Invert the input. 0 gives 1. 1 gives 0.	SHR - Shift bits right and discard rightmost bit.
Mod - Remainder after division
Inc - Increment (add one)
Dec - Decrement (subtract one)

COMMANDS	DIRECT EXAMPLES
OP	Assembler	Machine Code	Explanation
ADD	ADD AL,BL	A0 00 01	Add BL to AL
SUB	SUB CL,DL	A1 02 03	Subtract DL from CL
MUL	MUL AL,CL	A2 00 02	Multiply AL by CL
DIV	DIV BL,DL	A3 01 03	Divide BL by DL
MOD	MOD DL,BL	A6 03 01	Remainder after dividing DL by BL
INC	INC AL	A4 00	Add one to AL
DEC	DEC BL	A5 01	Deduct one from BL
AND	AND CL,AL	AA 02 00	CL becomes CL AND AL
OR	OR CL,DL	AB 02 03	CL becomes CL OR DL
XOR	XOR BL,AL	AC 01 00	BL becomes BL XOR AL
NOT	NOT CL	AD 02	Invert the bits in CL
ROL	ROL DL	9A 03	Bits in DL rotated one place left
ROR	ROR AL	9B 00	Bits in AL rotated one place right
SHL	SHL BL	9C 01	Bits in BL shifted one place left
SHR	SHR CL	9D 02	Bits in CL shifted one place right

COMMANDS	IMMEDIATE EXAMPLES
OP	Assembler	Machine Code	Explanation
ADD	ADD AL,15	B0 00 15	Add 15 to AL
SUB	SUB BL,05	B1 01 05	Subtract 5 from BL
MUL	MUL AL,10	B2 00 10	Multiply AL by 10
DIV	DIV BL,04	B3 01 04	Divide BL by 4
MOD	MOD DL,20	B6 03 20	Remainder after dividing DL by 20
AND	AND CL,55	BA 02 55	CL becomes CL AND 55 (01010101)
OR	OR CL,AA	BB 02 AA	CL becomes CL OR AA (10101010)
XOR	XOR BL,F0	BC 01 F0	BL becomes BL XOR F0

Examples

ADD CL,AL - Add CL to AL and put the answer into CL.

ADD AL,22 - Add 22 to AL and put the answer into AL.

The answer always goes into the first register in the command.

Data Movement Direction Image

DEC BL - Subtract one from BL and put the answer into BL.

The other commands all work in the same way.

Flags

If a calculation gives a zero answer, set the Z zero flag.
If a calculation gives a negative answer, set the S sign flag.
If a calculation overflows, set the O overflow flag.

An overflow happens if the result of a calculation has more bits than will fit into the available register. With 8 bit registers, the largest numbers that fit are -128 to + 127.

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