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pea.md

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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
- - - - -

Source operand addressing modes

Dn An (An) (An)+ ‑(An) (d,An) (d,An,Xi) ABS.W ABS.L (d,PC) (d,PC,Xn) imm
✓ ✓ ✓ ✓ ✓ ✓ ✓

From MOTOROLA M68000 FAMILY Programmer's reference manual. Copyright 1992 by Motorola Inc./NXP. Adapted with permission.