Motorola 68000

45
The 68000's Instruction Set
Application: Used to set bits in the SR (i.e., the S, T, and interrupt mask bits).
For example, ORI #$8000,SR sets bit 15 of the SR (i.e., the trace
bit).
Condition codes: X N Z V C
* * * * *
X is set if bit 4 of data = 1; unchanged otherwise
N is set if bit 3 of data = 1; unchanged otherwise
Z is set if bit 2 of data = 1; unchanged otherwise
V is set if bit 1 of data = 1; unchanged otherwise
C is set if bit 0 of data = 1; unchanged otherwise
PEA Push effective address
Operation: [SP] ← [SP] - 4; [M([SP])] ← <ea>
Syntax: PEA <ea>
Attributes: Size = longword
Description: The longword effective address specified by the instruction is
computed and pushed onto the stack. The difference between
PEA and LEA is that LEA calculates an effective address and puts it
in an address register, while PEA calculates an effective address
in the same way but pushes it on the stack.
Application: PEA calculates an effective address to be used later in address
register indirect addressing. In particular, it facilitates the writing
of position independent code. For example, PEA (TABLE,PC)
calculates the address of TABLE with respect to the PC and pushes
it on the stack. This address can be read by a procedure and then
used to access the data to which it points. Consider the example:
PEA Wednesday Push the parameter address on the stack
BSR Subroutine Call the procedure
LEA (4,SP),SP Remove space occupied by the parameter
.
Subroutine MOVEA.L (4,SP),A0 A0 points to parameter under return address
MOVE.W (A0),D2 Access the actual parameter  Wednesday
.
RTS
Condition codes: X N Z V C
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