Motorola 68000 Guida all'installazione

Motorola 68000's

Instruction Set

We have included this appendix to save you the task of having to turn to secondary

material when writing 68000 assembly language programs. Since most

programmers are not interested in the encoding of instructions, details of

instruction encoding have been omitted (i.e., the actual op-code bit patterns).

Applications of some of the instructions have been provided to demonstrate how

they can be used in practice.

Instructions are listed by mnemonic in alphabetical order. The information

provided about each instruction is: its assembler syntax, its attributes (i.e., whether

it takes a byte, word, or longword operand), its description in words, the effect its

execution has on the condition codes, and the addressing modes it may take. The

effect of an instruction on the CCR is specified by the following codes:

UThe state of the bit is undefined (i.e., its value cannot be predicted)

-The bit remains unchanged by the execution of the instruction

*The bit is set or cleared according to the outcome of the instruction.

Unless an addressing mode is implicit (e.g., NOP, RESET, RTS, etc.), the legal

source and destination addressing modes are specified by their assembly language

syntax. The following notation is used to describe the 68000's instruction set.

Dn, An Data and address register direct.

(An) Address register indirect.

(An)+, -(An) Address register indirect with post-incrementing or pre-

decrementing.

(d,An), (d,An,Xi) Address register indirect with displacement, and address

ABS.W, ABS.L Absolute addressing with a 16-bit or a 32-bit address.

(d,PC), (d,PC,Xi) Program counter relative addressing with a 16-bit offset,

or with an 8-bit offset plus the contents of an index

imm An immediate value (i.e., literal) which may be 16 or 32

bits, depending on the instruction.

The 68000's Instruction Set

2The 68000's Instruction Set

Two notations are employed for address register indirect addressing. The

notation originally used to indicate address register indirect addressing has been

superseded. However, the Teesside 68000 simulator supports only the older form.

Old notation Current notation

d(An), d(An,Xi) (d,An), (d,An,Xi)

d(PC), d(PC,Xi) (d,PC), (d,PC,Xi)

ABCD Add decimal with extend

Operation: [destination]10 ←[source]10 + [destination]10 + [X]

Syntax: ABCD Dy,Dx

ABCD -(Ay),-(Ax)

Attributes: Size = byte

Description: Add the source operand to the destination operand along with

the extend bit, and store the result in the destination location.

The addition is performed using BCD arithmetic. The only legal

addressing modes are data register direct and memory to memory

with address register indirect using pre-decrementing.

Application: The ABCD instruction is used in chain arithmetic to add together

strings of BCD digits. Consider the addition of two nine-digit

numbers. Note that the strings are stored so that the least-

significant digit is at the high address.

LEA Number1,A0 A0 points at first string

LEA Number2,A1 A1 points at second string

MOVE #8,D0 Nine digits to add

MOVE #$04,CCR Clear X-bit and Z-bit of the CCR

LOOP ABCD -(A0),-(A1) Add a pair of digits

DBRA D0,LOOP Repeat until 9 digits added

Condition codes: X N Z V C

* U * U *

The Z-bit is cleared if the result is non-zero, and left unchanged

otherwise. The Z-bit is normally set by the programmer before

the BCD operation, and can be used to test for zero after a chain

of multiple-precision operations. The C-bit is set if a decimal

carry is generated.

The 68000's Instruction Set

ADD Add binary

Operation: [destination] ←[source] + [destination]

Syntax: ADD <ea>,Dn

ADD Dn,<ea>

Attributes: Size = byte, word, longword

Description: Add the source operand to the destination operand and store the

result in the destination location.

Condition codes: X N Z V C

* * * * *

Source operand addressing modes

Destination operand addressing modes

ADDA Add address

Operation: [destination] ←[source] + [destination]

Syntax: ADDA <ea>,An

Attributes: Size = word, longword

Description: Add the source operand to the destination address register and

store the result in the destination address register. The source is

sign-extended before it is added to the destination. For example,

if we execute ADDA.W D3,A4 where A4 = 0000010016 and D3.W =

800216, the contents of D3 are sign-extended to FFFF800216 and

added to 0000010016 to give FFFF810216, which is stored in A4.

4The 68000's Instruction Set

Application: To add to the contents of an address register and not update the

CCR. Note that ADDA.W D0,A0 is the same as LEA (A0,D0.W),A0.

Condition codes: X N Z V C

- - - - -

An ADDA operation does not affect the state of the CCR.

Source operand addressing modes

ADDI Add immediate

Operation: [destination] ←<literal> + [destination]

Syntax: ADDI #<data>,<ea>

Attributes: Size = byte, word, longword

Description: Add immediate data to the destination operand. Store the result

in the destination operand. ADDI can be used to add a literal

directly to a memory location. For example, ADDI.W #$1234,$2000

has the effect [M(200016)]← [M(200016)] +123416.

Condition codes: X N Z V C

* * * * *

Destination operand addressing modes

ADDQ Add quick

Operation: [destination] ←<literal> + [destination]

Syntax: ADDQ #<data>,<ea>

The 68000's Instruction Set

Sample syntax: ADDQ #6,D3

Attributes: Size = byte, word, longword

Description: Add the immediate data to the contents of the destination operand.

The immediate data must be in the range 1 to 8. Word and

longword operations on address registers do not affect condition

codes. Note that a word operation on an address register affects

all bits of the register.

Application: ADDQ is used to add a small constant to the operand at the effective

address. Some assemblers permit you to write ADD and then choose

ADDQ automatically if the constant is in the range 1 to 8.

Condition codes: Z N Z V C

* * * * *

Note that the CCR is not updated if the destination operand is an

address register.

Destination operand addressing modes

ADDX Add extended

Operation: [destination] ←[source] + [destination] + [X]

Syntax: ADDX Dy,Dx

ADDX -(Ay),-(Ax)

Attributes: Size = byte, word, longword

Description: Add the source operand to the destination operand along with

the extend bit, and store the result in the destination location.

The only legal addressing modes are data register direct and

memory to memory with address register indirect using pre-

decrementing.

Application: The ADDX instruction is used in chain arithmetic to add together

strings of bytes (words or longwords). Consider the addition of

6The 68000's Instruction Set

two 128-bit numbers, each of which is stored as four consecutive

longwords.

LEA Number1,A0 A0 points at first number

LEA Number2,A1 A1 points at second number

MOVE #3,D0 Four longwords to add

MOVE #$00,CCR Clear X-bit and Z-bit of the CCR

LOOP ADDX -(A0),-(A1) Add pair of numbers

DBRA D0,LOOP Repeat until all added

Condition codes: X N Z V C

* * * * *

The Z-bit is cleared if the result is non-zero, and left unchanged

otherwise. The Z-bit can be used to test for zero after a chain of

multiple precision operations.

AND AND logical

Operation: [destination] ←[source].[destination]

Syntax: AND <ea>,Dn

AND Dn,<ea>

Attributes: Size = byte, word, longword

Description: AND the source operand to the destination operand and store

the result in the destination location.

Application: AND is used to mask bits. If we wish to clear bits 3 to 6 of data

the AND operation cannot be used with an address register as

either a source or a destination operand. If you wish to perform a

logical operation on an address register, you have to copy the

address to a data register and then perform the operation there.

Condition codes: X N Z V C

- * * 0 0

Source operand addressing modes

The 68000's Instruction Set

Destination operand addressing modes

ANDI AND immediate

Operation: [destination] ←<literal>.[destination]

Syntax: ANDI #<data>,<ea>

Attributes: Size = byte, word, longword

Description: AND the immediate data to the destination operand. The ANDI

permits a literal operand to be ANDed with a destination other

than a data register. For example, ANDI #$FE00,$1234 or

ANDI.B #$F0,(A2)+.

Condition codes: X N Z V C

- * * 0 0

Destination operand addressing modes

ANDI to CCR AND immediate to condition

code register

Operation: [CCR] ←<data>.[CCR]

Syntax: ANDI #<data>,CCR

Attributes: Size = byte

Description: AND the immediate data to the condition code register (i.e., the

least-significant byte of the status register).

8The 68000's Instruction Set

Application: ANDI is used to clear selected bits of the CCR. For example,

ANDI #$FA,CCR clears the Z- and C-bits, i.e., XNZVC = X N 0 V 0.

Condition codes: X N Z V C

* * * * *

X: cleared if bit 4 of data is zero

N: cleared if bit 3 of data is zero

Z: cleared if bit 2 of data is zero

V: cleared if bit 1 of data is zero

C: cleared if bit 0 of data is zero

ANDI to SR AND immediate to status register

Operation: IF [S] =1

THEN

[SR] ←<literal>.[SR]

ELSE TRAP

Syntax: ANDI #<data>,SR

Attributes: Size = word

Description: AND the immediate data to the status register and store the

result in the status register. All bits of the SR are affected.

Application: This instruction is used to clear the interrupt mask, the S-bit, and

the T-bit of the SR. ANDI #<data>,SR affects both the status byte

of the SR and the CCR. For example, ANDI #$7FFF,SR clears the

trace bit of the status register, while ANDI #$7FFE,SR clears the

trace bit and also clears the carry bit of the CCR.

Condition codes: X N Z V C

* * * * *

ASL,ASR Arithmetic shift left/right

Operation: [destination] ←[destination] shifted by <count>

Syntax: ASL Dx,Dy

ASR Dx,Dy

ASL #<data>,Dy

ASR #<data>,Dy

ASL <ea>

ASR <ea>

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