[Sdcc-user] pic16: Efficient combining of 8-bit SFRs into a single 16-bit variable?

[Oberholtzer,Stephen <ste...@fr...>]

Most PICs have a 16-bit timer (named Timer1). The PIC18-series enhances
Timer1 (and its other 16-bit timers) to offer the choice of a consistent
read of the 16-bit value (lessening the pain of accurately  reading a
16-bit volatile value 8 bits at a time): a bit in the timer's control
register enables a latched read of the high-byte when the low-byte is
read by the CPU, so that you can do this:

movwf TMR1L	; when setup properly, at the same moment TMR1L is read,
a copy of TMR1H is latched into a buffer
; do stuff
movwf TMR1H ; reads the value that was latched into the buffer when
TMR1L was read
; do more stuff


It seems to me that constructing a 16-bit variable from two 8-bit
registers should be fairly simple, considering that the 16-bit variable
is, itself, two 8-bit memory locations.  So I'm kinda surprised to see
this code generated when I tried this:

sdcc -mpic16 -p18f452 -c makeword.c


;	.line	11; makeword.c	_tmr1 =3D (TMR1L)|(TMR1H<<8);
	MOVFF	_TMR1H, r0x00
	CLRF	r0x01
	MOVF	r0x00, W
	MOVWF	r0x03
	CLRF	r0x02
	MOVFF	_TMR1L, r0x00
	CLRF	r0x01
	MOVF	r0x02, W
	IORWF	r0x00, W
	BANKSEL	__tmr1
	MOVWF	__tmr1, B
	MOVF	r0x03, W
	IORWF	r0x01, W
	BANKSEL	(__tmr1 + 1)
	MOVWF	(__tmr1 + 1), B


I noticed that SDCC pays no mind to the order of the statement:
(TMR1L)|(TMR1H<<8) and (TMR1H<<8)|(TMR1L); both yield identical code.  I
don't know what the C standard has to say about this, wrt. volatiles,
but I'm sure it's OK. (incidentally, can someone point me to a copy of
the C99 standard?)

In any case, I would expect something more like this:

movff _TMR1L, (__tmr1)
movff _TMR1H, (__tmr1 + 1)

Obviously I'm going about this the wrong way.  What would be the proper
way to tell SDCC what I want it to do?


This E-Mail is intended only for the use of the individual or entity to =
which it is addressed, and may contain information that is privileged, =
confidential and exempt from disclosure under applicable law.  If you =
have received this communication in error, please do not distribute it =
and notify us immediately by telephone: 610-902-9000 and delete the =
original message.