Motorola M68000 User's Guide download pdf (Page 125)

Subroutines 111

HOVEA.L A6,SP

RTS

All of these operations are made simpler by using the LINK and

UNLK instructions. These have the added advantage that the register

used as the frame pointer is automatically saved and restored. The LINK

instruction has the general form

LINK A n r#<<3i6placement>

where <displacement> is the value to be added to the stack pointer.

Normally a negative displacement is used to allocate locals. The LINK

instruction first pushes An on the stack, and then loads An with the

updated value. Finally, the displacement is added to SP. The result will

look almost like our example above, except that An will be on the stack.

Here is what a frame will look like after a LINK instruction.

High

Lou

arguments

return address

saved fin

locals

temporaries

fin (Frame Pointer)

Naturally, the offsets to the parameters are slightly different. The first

parameter is now found at 8(An) rather than 4(An).

The unlink instruction, UNLK, merely loads SP from An and pops the

saved An from its location on the stack. As a final example, let’s write a

subroutine, GAMMA, that uses LINK and UNLK and requires 100 bytes

of local storage.

1 2 ... 120 121 122 123 124 125 126 127 128 129 130 ... 255 256

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Motorola M68000 User's Guide Page 125