OpenEuphoria

RE: Damn Unions

• Posted by jordah ferguson <jorfergie03 at yahoo.com> Jul 30, 2002
• 520 views

Hi Derek,

merci beaucoup!

I was hoping you would help me out though yours came through last. Thank 
you

jordah
Derek Parnell wrote:
> 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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> 
> 
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> 
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