The msp430 requires 16-bit aligned addresses for 16-bit operations. In the MSP430 x1xx User's Guide, Section 1.4.5 RISC 16-bit CPU, Instruction Set, Memory Organization,
"Bytes are located at even or odd addresses. Words are only located at even addresses as shown in Figure 1-3. When using word instructions, only even addresses may be used. The low byte of a word is always an even address. The high byte is at the next odd address." In the MSPGCC Manual in section "Byte and word issues" http://mspgcc.sourceforge.net/manual/x214.html: "Word operands must be located at even addresses. ... The processor's behaviour when a word is accessed at an odd location is poorly documented. In all current processors the lower bit is just silently ignored. The effect is, therefore, the same as specifying an address which is one less." This is why the packed attribute is very, very bad for the msp430. Your 32-bit args value is placed at an odd-value address with the packed attribute. When given that odd address, the msp430 quietly converts it to the next lower even address, thus overwriting your type variable. If your structure must be 5 bytes and byte aligned, use nesC's network types (nx_uint8_t, nx_uint32_t, etc) and leave out the packed attribute. Cory On 2/17/06, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > Hi, > > I have a problem when using __attribute((packed)) for a struct under > msp430 (telosb). > This is the struct I have > > typedef struct{ > uint32_t interval; > }start_args_t; > > typedef struct{ > uint32_t time; > }set_time_args_t; > > typedef struct{ > uint8_t type; > union{ > start_args_t startArgs; > set_time_args_t setTimeArgs; > }args; > }Cmd_t __attribute((packed)) ; > > Without the "packed" attribute the struct has a sizeof 6 and padding in > this way > > type 1 byte > 0x00 padding byte > args 4 bytes > > With the "packed" attibute the struct has a sizeof 5, but the variable > type and the args starts at the same address. I mean that if I set first > the args and then the type field, I am rewriting the LSB of the args > field. > > If I change the struct to this one > > typedef struct{ > union{ > start_args_t startArgs; > set_time_args_t setTimeArgs; > }args; > uint8_t type; > }Cmd_t __attribute((packed)) ; > > The "packet" attribute behaves well. It seems like if the odd field are at > the end of the struct de packing behaves well. > > I wonder if this a compiler related bug or I am missing something. > > thanks in advance, > > -Bill > > _______________________________________________ > Tinyos-help mailing list > [email protected] > https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help > _______________________________________________ Tinyos-help mailing list [email protected] https://mail.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
