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 following: Addressing Mode Mode Register Dn - - An - - (An) 010 No reg. An (An)+ 011 No reg. An -(An) 100 No reg. An (d16,An) 101 No reg. An (d8,An,Xi) 110 No reg. An (bd,An,Xi) 110 No reg. An ([bd,An,Xi]od) 110 No reg. An ([bd,An],Xi,od) 110 No reg. An Addressing Mode Mode Register Abs.W 111 000 Abs.L 111 001 (d16,PC) - - (d8,PC,Xi) - - (bd,PC,Xi) - - ([bd,PC,Xi],od) - - ([bd,PC],Xi,od) - - #data - -
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.

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