xch	a, pulse_l		; save A
	clr	a
	xch	a, th0			; read and clear  TH=01 TL=ff
	mov	period_l, a
	clr	a
	xch	a, tl0			; read and clear 01
	xch	a, pulse_l

will read 0101 regardless of xch or mov!

Since an overflow occur only on counting FF->00, but never on loading 00 into the low byte regardless the state prior loading.
It does not matter what you load into the timer the overflow result from the clocking

I am totally convinced that the above will read wrong in the case I describe, if you want to refute it, please do so with the timer values I use.

Erik