Comparing and Branching Operations

UMBC CMSC 313

The comparisons will be done in the FPU and the decision to branch will be done in the CPU. This must be done in two steps.

**FPU Compare Instructions**
fcom[p]	mem32 or mem64	; compare ST0 to operand [and pop]
fcom[p]	STn	; compare ST0 to STn [and pop]
fcom[p]		; compare ST0 to ST1 [and pop]
fcompp		; compare ST0 to ST1 and pop twice
ficom[p]	mem16 or mem32	; compare ST0 to converted integer operand [and pop]
ftst		; compare ST0 to 0.0

The FPU will compare and set bits in an FPU register (status word). These must then be transferred to the Flags Register in the CPU.

StatWd	DW	?	; defined in the .DATA Segment
	; ...
	fld	B4
	fld	C4
	fcom
	fstsw	StatWd	; Store status word in StatWd
	mov	ax, StatWd
	sahf		Transfer ah into flags register
	jbe	OrderOK	; Note unsigned jump
	fxch
OrderOK
	fstp	A4
	fcomp	ST	; pop smaller item