Compare And Swap

NAME
	CAS -- Compare and swap	(68020+)

SYNOPSIS
	CAS	Dc,Du,<ea>

	Size = (Byte, Word, Long)

FUNCTION
	This instruction is a read-modify-write instruction and should
	NEVER be used on Amiga because of conflicts with customs chips.
	Destination operand, which is in memory at address specified
	by <ea>, is compared to  data register Dc (Data Compare). This
	register is used as reference in the principle of this instruction.
	If there's equality (Z=1), destination operand can be updated, i.e.
	the new operand Du (Data Update) is moved in destination.
	If there's no equality (Z=0), it's the reference register Dc which
	must be updated. So there's a move from destination operand to Dc.

FORMAT
                                                          <ea>
	----------------------------------------=========================
	|15 |14 |13 |12 |11 |10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
	|---|---|---|---|---|-------|---|---|---|-----------|-----------|
	| 0 | 0 | 0 | 0 | 1 | SIZE  | 0 | 1 | 1 |   MODE    |  REGISTER |
	|---|---|---|---|---|-------|-----------|-----------|-----------|
	| 0 | 0 | 0 | 0 | 0 | 0 | 0 |Du REGISTER| 0 | 0 | 0 |Dc REGISTER|
	-----------------------------------------------------------------

SIZE
	01->one Byte operation
	10->one Word operation
	11->one Long operation

REGISTER
	Du register: indicates number of data register, which contains the
	new value to update in destination operand.

	Dc register: indicates number of data register, which contains the
	reference value to compare to destination operand.

	<ea> is always destination, addressing modes are the followings:
	--------------------------------- -------------------------------
	|Addressing Mode|Mode| Register | |Addressing Mode|Mode|Register|
	|-------------------------------| |-----------------------------|
	|      Dn       | -  |     -    | |    Abs.W      |111 |  000   |
	|-------------------------------| |-----------------------------|
	|      An       | -  |     -    | |    Abs.L      |111 |  001   |
	|-------------------------------| |-----------------------------|
	|     (An)      |010 |N reg. An| |   (d16,PC)    | -  |   -    |
	|-------------------------------| |-----------------------------|
	|     (An)+     |011 |N reg. An| |   (d8,PC,Xi)  | -  |   -    |
	|-------------------------------| |-----------------------------|
	|    -(An)      |100 |N reg. An| |   (bd,PC,Xi)  | -  |   -    |
	|-------------------------------| |-----------------------------|
	|    (d16,An)   |101 |N reg. An| |([bd,PC,Xi],od)| -  |   -    |
	|-------------------------------| |-----------------------------|
	|   (d8,An,Xi)  |110 |N reg. An| |([bd,PC],Xi,od)| -  |   -    |
	|-------------------------------| |-----------------------------|
	|   (bd,An,Xi)  |110 |N reg. An| |    #data      | -  |   -    |
	|-------------------------------| -------------------------------
	|([bd,An,Xi]od) |110 |N reg. An|
	|-------------------------------|
	|([bd,An],Xi,od)|110 |N reg. An|
	---------------------------------

RESULT
	X - not affected
	N - Set if the result of comparison is negative. Cleared otherwise.
	Z - Set if the result of comparison is zero. Cleared otherwise.
	V - Set if overflow. Cleared otherwise.
	C - Set if carry. Cleared otherwise.

SEE ALSO
	CAS2

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