topic Struct members memory order in 8-bit Microcontrollers

Struct members memory order

S_B_D — Mon, 09 Mar 2009 23:23:54 GMT

Hi,

Is there a way to tell the linker to place struct members in memory in the order they where defined in the code? This will place the struct members in consecutive order as written in the struct definition.

The reason for this is to use the memcpy function to copy an entire memory segment ("as is") into the struct.

Thanks,

S.B.D

Re: Struct members memory order

CrasyCat — Mon, 09 Mar 2009 23:44:21 GMT

Hello

As far as I could check structure fields are allocated in the sequence they are defined.

- Which CPU are you targeting (HC08, HC12, Coldfire, ..)
- Which version of CodeWarrior are you using?
To retrieve that info:
- Start CodeWarrior
- Select Help -> About Freescale CodeWarrior
- Click on "Install Products"
- CodeWarrior version used is displayed on top in the Installed Products dialog.

Additionally how is the structure defined?

CrasyCat

Re: Struct members memory order

S_B_D — Tue, 10 Mar 2009 00:06:03 GMT

Thanks for the reply CrasyCat.

I'm working on the MC9S08JM32 on Code-Warrior 5.9.0 build 2830.

As you will see the structure is not aligned:

typedef struct{

unsigned char Misc;

unsigned int ConnectedSensors[8];

unsigned char   Timer1Prescaler;
unsigned int   Timer1AuxCounter;
unsigned int   Timer1Modulo;

unsigned char   NumberOfSamplesInPacket;
unsigned int   NumberOfSamplesInPacketToPc;
unsigned char PcRate;
unsigned int   TriggerLevelHigh;
unsigned int    TriggerLevelLow;

unsigned char TriggerType_Input;

unsigned int Calibration[8];

unsigned char Timing[4];
}Status;
extern Status STATUS, *STATUSptr;

S.B.D

Re: Struct members memory order

CompilerGuru — Tue, 10 Mar 2009 00:47:25 GMT

ANSI C requires that struct members are allocated in the declaration order, why do you think this is not happening here?

For copying structs, memcpy should work however just assigning structs is even easier.

Also not sure what your comment about alignment is about, alignment is a different issue than the order which fields are allocated. For a 8 bit chip as a S08, there is usually no alignment padding added by the compiler. In general, alignment is compiler specific and portable code should not rely on (the absence of) alignment.

Daniel