OpenEuphoria

RE: Damn Unions

• Posted by Derek Parnell <Derek.Parnell at SYD.RABOBANK.COM> Jul 26, 2002
• 388 views

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A Structure is the way we let the compiler know how we want a piece of RAM
used. It describes the layout of the memory area in terms of how the
compiler is supposed to store and fetch bytes from there. A Union is a way
of letting the compiler know that the same RAM location can be used
(described) in different ways. Normally a given RAM location contains data
that is used in only one way. However, occasionally there are times when we
can use the same area in RAM different ways.

So,

     union { 
         DWORD  dwOemId; 
         struct { 
             WORD wProcessorArchitecture; 
             WORD wReserved; 
         }; 

is saying that this piece of RAM can be thought of as either a single DWORD
or a single struct that contains two WORD items. For the purpose of mapping
this into Euphoria, the key is to locate the item in the union that is the
longest. This is then used to calculate the offset of the item that comes
after the union.

  constant  -- typedef struct _SYSTEM_INFO { // sinf 
     -- start of union
     SI_dwOemID     = 0,    -- length 4
     SI_wProcessorArchitecture = 0,  -- length 2, offset same as OemId
     SI_wReserved   = 2, -- length 2
     -- end of union
     SI_dwPageSize = 4, -- length 4
     SI_lpMinimumApplicationAddress = 8, -- length 4 
     SI_lpMaximumApplicationAddress = 12, -- length 4 
     SI_dwActiveProcessorMask = 16, -- length 4 
     SI_dwNumberOfProcessors = 20, -- length 4; 
     SI_dwProcessorType = 24, -- length 4; 
     SI_dwAllocationGranularity = 28, -- length 4; 
     SI_wProcessorLevel = 32, -- length 2; 
     SI_wProcessorRevision = 34, -- length 2; 
     SIZEOF_SYSTEM_INFO = 36 

------------
hth,
Derek

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