Inside Macintosh: PowerPC Numerics / Part 3 - Numerics in PowerPC Assembly Language

Chapter 12 - Assembly-Language Environmental Controls

Saving and Restoring the Floating-Point Environment

To save and restore the state of the entire floating-point environment, use the mffs and mtfsf instructions.

The mffs instruction saves the FPSCR to a floating-point register. It has the form

mffs   DST 

where DST is the floating-point register into which the FPSCR should be copied. For example, the instruction

mffs  f0

saves the current state of the FPSCR register in bits 32 through 63 of floating-point register F0. Bits 0 through 31 of register F0 are set to 1's.

To restore a floating-point environment that you have previously saved, use the mtfsf instruction. This instruction copies a 4-bit field from a floating-point register into an FPSCR field. It has the form

mtfsf     DST, SRC 

where DST is a 4-bit FPSCR field and SRC is the floating-point register from which the field should be copied. The instruction assumes that the last half of the floating-point register SRC contains an FPSCR value. Thus, if you specify

mtfsf    3,f0

bits 44 through 47 of register F0 are copied into FPSCR field 3, bits 12 through 15.
Figure 12-3 shows how the FPSCR fields map to a floating-point register.

Figure 12-3 SRC and DST fields for mtfsf instruction

Listing 12-3 saves the floating-point environment and then restores it.

Listing 12-3 Saving and restoring the floating-point environment

mffs		f10			# FPSCR copied into register f10

# other floating-point computations occur here

mtfsf		0,f10			# restore bits 0 and 3
mtfsf		1,f10			# restore bits 4 through 7
mtfsf		2,f10			# restore bits 8 through 11
mtfsf		3,f10			# restore bits 12 through 15
mtfsf		4,f10			# restore bits 16 through 19
mtfsf		5,f10			# restore bits 20 through 23
mtfsf		6,f10			# restore bits 24 through 27
mtfsf		7,f10			# restore bits 28 through 31
					# entire FPSCR now restored