Motorola M68000 User Manual download pdf (Page 100)

Floating Point Instructions

MOTOROLA M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL 5-89

FMOVEM Move Multiple Floating-Point FMOVEM

Data Registers

(MC6888X, MC68040)

In this example, a convention is defined by which each called procedure is passed a

word mask in D7 that identifies all floating-point registers in use by the calling proce-

dure. Bits 15 – 8 identify the registers in the order FP0 – FP7, and bits 7 – 0 identify the

registers in the order FP7 – FP0 (the two masks are required due to the different trans-

fer order used by the predecrement and postincrement addressing modes). The code

used by the calling procedure consists of simply moving the mask (which is generated

at compile time) for the floating-point data registers currently in use into D7:

Calling procedure...

The entry code for all other procedures computes two masks. The first mask identifies

the registers in use by the calling procedure that are used by the called procedure (and

therefore saved and restored by the called procedure). The second mask identifies the

registers in use by the calling procedure that are used by the called procedure (and

therefore not saved on entry). The appropriate registers are then stored along with the

two masks:

Called procedure...

If the second procedure calls a third procedure, a register mask is passed to the third

procedure that indicates which registers must not be altered by the third procedure.

This mask identifies any registers in the list from the first procedure that were not saved

by the second procedure, plus any registers used by the second procedure that must

not be altered by the third procedure.

MOVE.W #ACTIVE,D7 Load the list of FP registers that are

in use.

BSR PROC_2

MOVE.W D7,D6 Copy the list of active registers.

AND.W #WILL_USE,D7 Generate the list of doubly-used

registers.

FMOVEM D7, – (A7) Save those registers.

MOVE.W D7, – (A7) Save the register list.

EOR.W D7,D6 Generate the list of not saved active

registers.

MOVE.W D6, – (A7) Save it for later use.

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Motorola M68000 User Manual Page 100