PBC equivelent to a VAR Overlay

[DaveC3]

I am converting a program from Bascom Basic to PBP. They have method of defining a VAR with a OVERLAY postscript.

Here is their definition of Overlay


OVERLAY

The OVERLAY option will not use any variable space. It will create a sort of phantom variable:



Dim x as Long at $60 'long uses 60,61,62 and 63 hex of SRAM



Dim b1 as Byte at $60 OVERLAY

Dim b2 as Byte at $61 OVERLAY



B1 and B2 are no real variables! They refer to a place in memory. In this case to &H60 and &H61. By assigning the phantom variable B1, you will write to memory location &H60 that is used by variable X.

So to define it better, OVERLAY does create a normal usable variable, but it will be stored at the specified memory location which could be already be occupied by another OVERLAY variable, or by a normal variable.


How would I go about translating this.

Thanks

Dave

[HenrikOlsson]

Section 4.4 in the manual....

x VAR LONG '32 bit variable - takes up actual RAM
b1 VAR x.BYTE0 'Alias to byte 0.
b2 VAR x.BYTE1
b3 VAR x.BYTE2
b4 VAR x.BYTE.3
w1 VAR x.WORD0 'Alias to the low word of x
w2 VAR x.WORD1 'Alias to the high word of x

/Henrik.

[DaveC3]

Thanks Henrik

so to translate from




Dim MyVar As Byte At Buffer + &H01 Overlay



I would use




MyVar VAR Buffer.Byte$01



Thanks again

Dave

[HenrikOlsson]

Hi,
No, in that case and as long as Buffer is a WORD (2bytes) or a LONG (4bytes) you can use:
MyVar VAR Buffer.Byte1

If Buffer is an array of BYTES ie.
Buffer VAR BYTE[128]
Then you need to index that array, like:
MyVar1 VAR Buffer[0]
MyVar2 VAR Buffer[1]
MyVar3 VAR Buffer[75]

And so on.

/Henrik.

[DaveC3]

Thanks Henrik

I got it.

Dave