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Index > DOS > CWSDPMI and 0x100 allocate DOS memory

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hodbogi



Joined: 14 Oct 2020
Posts: 2
hodbogi
Hello, I have been looking through some examples and resources both here and anywhere I can find and I am not quite sure why my code does not appear to work.

I am trying to test allocating DPS memory and writing to it (just 0's for now) but every time I allocate the page and then write to it I get a general protection fault. Looking at the DPMI 1.0 spec it says I am not supposed to modify any LDT information for DOS allocations, so I avoided that.

IO am using CWSDPMI and the code is as follows:

Code:
mov ax, 0x100 ; Allocate DOS memory
mov bx, 2 ; 32 bytes
int 0x31 ; DPMI
jc . end ; Error = carry set

mov es, dx ; selector is in dx
mov di, ax ; Offset is in ax
mov cx, 10 ; Just 10 bytes to copy for now
mov al, 0 ; Fill with 0s.
cld ; Increment di + 1 per rep
rep stosb ; Store string data

.end: ; Somewhere fuirther down in the code.

    


The general protection fault I have found happens exactly at the rep stosb line. I must be missing something?
Note: If there are typos in the code it is likely because I retyped all of this looking at my other computer screen.
Post 20 Oct 2020, 23:55
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sinsi



Joined: 10 Aug 2007
Posts: 709
Location: Adelaide
sinsi
DX is the pmode selector
AX is the real mode segment

The offset should be 0
Post 21 Oct 2020, 00:59
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hodbogi



Joined: 14 Oct 2020
Posts: 2
hodbogi
Ah that is actually making more sense now, I'm more of a 65xx / 68xx kind of person so I'm half confused about protected mode selectors and real mode segments, etc. I'm assuming it's not as simple as throwing ax into es and 0 into di and using stosb from there, right? that causes a protection fault as well. Do I have to convert it into a physical address or something somehow and go from there? By the nature of what I read about how stosb works, it doesn't seem that way. I'd hate to ask but I would definitely appreciate an example that I can study of how this would work in particular with function 0x100 like used here (a couple examples I looked at online to accomplish certain tasks used it), so that I can learn a little more about that aspect.
Post 21 Oct 2020, 16:11
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sinsi



Joined: 10 Aug 2007
Posts: 709
Location: Adelaide
sinsi
When you are using that memory, you are in protected mode and would use DX:0 to access it.
Usually you would allocate DOS memory to use with a real mode DOS or BIOS call, in that case you pass AX:0 as the memory address.
Post 21 Oct 2020, 19:43
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