Data types, signed vs. unsigned. Or confused vs. informed.

I am trying to read bytes from a file. So I open the file, read 1 byte, and read another byte.

Then I print the read bytes through my own logging functions. And I get bizarre numbers written to the log.

The first number read from the file is 0xB2 and is printed as -78 to the log. That is ok since the variable that holds the read byte is a signed 8 bit variable. But the next number that is read is stored as 0x80 in the file, but printed as -3 in the log. This is very strange to me. Why is it not printed as -128, as you’d expect?

In any case, while digging around in the LMS2012 source I found this snippet:

//        VM DATA TYPES

typedef   SBYTE                 DATA8;  //!< VM Type for 1 byte signed value
typedef   SWORD                 DATA16; //!< VM Type for 2 byte signed value
typedef   SLONG                 DATA32; //!< VM Type for 4 byte signed value
typedef   FLOAT                 DATAF;  //!< VM Type for 4 byte floating point value

//        VM VARIABLE TYPES

typedef   UBYTE             VARDATA;    //!< Variable base type
typedef   UBYTE             IMGDATA;    //!< Image base type

typedef   UWORD             PRGID;      //!< Program id type

typedef   UWORD             OBJID;      //!< Object id type
typedef   IMGDATA*          IP;         //!< Instruction pointer type
typedef   VARDATA*          LP;         //!< Local variable pointer type
typedef   VARDATA*          GP;         //!< global variable pointer type

typedef   ULONG             IMINDEX;    //!< ImageData index type
typedef   ULONG             GBINDEX;    //!< GlobalBytes index type
typedef   ULONG             LBINDEX;    //!< LocalBytes index type
typedef   UWORD             TRIGGER;    //!< TriggerCount type
typedef   UBYTE             PARS;       //!< NoOfParameters type
typedef   SLONG             IMOFFS;     //!< ImageData offset type

typedef   DATA16            HANDLER;    //!< Memory list index

So it seems there are no unsigned numbers exposed to the VM. So if I read 0xB2 from file, it will forever be stored as -78, even when moved from a signed 8 bit variable to a signed 16 bit variable. Unless I convert it myself of course, but there is no built in converting from signed to unsigned so I need to make my own functions to handle that too.

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