[Sdcc-user] Problem creating int from 2 bytes

[Bernie R. <ra...@sp...>]

Hello 

   I have been using the SDCC compiler and have had good results
with it. I have come across a problem that I can't understand. If
you could, I would be a great benefit if you could help me with an
answer. 

   I am using version version 2.3.0 of sdcc. I am having a problem
with creating a 16 bit interger from 2 8 bit bytes. I am writing
in C. below is the snippet of the working and non-working C and the
resulting assembly code generated by the asx8051 assembler. 


    As a summary of my problem, SETUPDAT[3] holds 0x1F and SETUPDAT[2]
holds 0xFF. I want to combine these two bytes into a word variable
that is at r2,r3 in order to produce 0x1FFF. The WORKING code does it
but the NOT WORKING code results in 0x1EFF. I have marked with an 
'*' the only lines of assembly code that are different. (I didn't
have much luck discovering what rlc followed by subb were doing.) 


WORKING:
            addr = SETUPDAT[3];
            addr <<= 8;
            addr += (SETUPDAT[2] & 0xFF);

                           1692 ;	src/ezusb.c 586
   067E                    1693 00162$:
   067E 90 7F EB           1694 	mov	dptr,#(_SETUPDAT + 0x0003)
   0681 E0                 1695 	movx	a,@dptr
                           1696 ; Peephole 105   removed redundant mov
   0682 FA                 1697 	mov  r2,a 
   0683 33                 1698 	rlc	a
   0684 95 E0              1699 	subb	a,acc
   0686 FB                 1700 	mov	r3,a 
                           1701 ;	src/ezusb.c 587
   0687 8A 03              1702 	mov	ar3,r2 
   0689 7A 00              1703 	mov	r2,#0x00 
                           1704 ;	src/ezusb.c 588
   068B 90 7F EA           1705 	mov	dptr,#(_SETUPDAT + 0x0002)
   068E E0                 1706 	movx	a,@dptr
   068F FC                 1707 	mov	r4,a     
*  0690 7D 00              1708 	mov	r5,#0x00 
   0692 EC                 1709 	mov	a,r4
   0693 2A                 1710 	add	a,r2
   0694 FA                 1711 	mov	r2,a     
   0695 ED                 1712 	mov	a,r5
   0696 3B                 1713 	addc	a,r3     
   0697 FB                 1714 	mov	r3,a


NOT WORKING: 
            addr = SETUPDAT[3];
            addr = addr << 8;
            addr += SETUPDAT[2];

                           1692 ;	src/ezusb.c 586
   067E                    1693 00162$:
   067E 90 7F EB           1694 	mov	dptr,#(_SETUPDAT + 0x0003)
   0681 E0                 1695 	movx	a,@dptr
                           1696 ; Peephole 105   removed redundant mov
   0682 FA                 1697 	mov  r2,a
   0683 33                 1698 	rlc	a
   0684 95 E0              1699 	subb	a,acc
   0686 FB                 1700 	mov	r3,a
                           1701 ;	src/ezusb.c 587
   0687 8A 03              1702 	mov	ar3,r2
   0689 7A 00              1703 	mov	r2,#0x00
                           1704 ;	src/ezusb.c 588
   068B 90 7F EA           1705 	mov	dptr,#(_SETUPDAT + 0x0002)
   068E E0                 1706 	movx	a,@dptr
                           1707 ; Peephole 105   removed redundant mov
   068F FC                 1708 	mov  r4,a
*   0690 33                 1709 	rlc	a
*   0691 95 E0              1710 	subb	a,acc
*   0693 FD                 1711 	mov	r5,a
   0694 EC                 1712 	mov	a,r4
   0695 2A                 1713 	add	a,r2
   0696 FA                 1714 	mov	r2,a
   0697 ED                 1715 	mov	a,r5
   0698 3B                 1716 	addc	a,r3
   0699 FB                 1717 	mov	r3,a

-- 
| Bernie Radecki       SA PGU R&D Firmware Engineer                  |
| Agilent Technologies                   e_mail: ra...@ag... |
| 24001 East Mission Ave., MS 3WU/222    phone:  (509)921-3655       |
| Liberty Lake, WA. 99019-9599           fax:    (509)921-3991       |
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