Comparing and Branching

UMBC CMSC 211

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 ST to operand [and pop]
fcom[p]	ST(n)	; compare ST to ST(n) [and pop]
fcom[p]		; compare ST to ST(1) [and pop]
fcompp		; compare ST to ST(1) and pop twice
ficom[p]	mem16 or mem32	; compare ST to converted integer operand [and pop]
ftst		; compare ST 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	B
	fld	C
	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	A
	fcomp	ST	; pop smaller item