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> DOS > Accessing low memory from DPMI |
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Japheth 29 Dec 2006, 23:24
> How to access low DOS memory from DPMI ?
It depends 
> Do I have full access to all the area from 0 to$FFFFF or even $10FFFF ? 0-FFFFFh yes, 0-10FFFFh not necessarily. > DGJPP provides the functions "dosmemget" and dosmemput" for this - > how to do in ASM ? Do you want access to conventional memory or do you want to allocate conventional memory. This is a tiny difference. To get/free/resize DOS memory, there exist some DPMI functions (0x100-0x102). > What is the best way to write a bigger amount of > data into VGA memory (80386 code, no MMX) ? "rep movsd" will be a good starting point . |
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29 Dec 2006, 23:24 |
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DOS386 30 Dec 2006, 01:33
Thanks.
Quote: 0-10FFFFh not necessarily. No problem 
Quote: Do you want access to conventional memory or do you want to allocate conventional memory. This is a tiny difference. Access only (VGA, "environment"). Is it accessible without previous creation/allocation of segments, "selectors", etc. ? Quote: "rep movsd" will be a good starting point How to tell the CPU to write into low memory instead my great 4 MB segment ? _________________ Bug Nr.: 12345 Title: Hello World program compiles to 100 KB !!! Status: Closed: NOT a Bug |
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30 Dec 2006, 01:33 |
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Japheth 30 Dec 2006, 08:11
> How to tell the CPU to write into low memory instead my great 4 MB segment ?
this is a weakness of the simple MZ file format approach: you can easily get a flat memory-model, but it is not zero-based (that's why PE or LE formats are a better choice for 32bit and why almost all DOS extenders [except DJGPP] use them). Best thing to do for MZ is to allocate another descriptor and make it zero-based, 4 GB in size. Then use FS or GS to permanently hold it. For memory transfers with "REP MOVSD" you mightl then have to change DS/ES temporarily. |
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30 Dec 2006, 08:11 |
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DOS386 31 Dec 2006, 03:36
Thanks.
Quote: (that's why PE or LE formats are a better choice for 32bit and why almost all DOS extenders [except DJGPP] use them) I have my reasons NOT to use extenders/compilers/linkers using/producing bloated executables full of garbage and external crappy code
Quote: this is a weakness of the simple MZ file format approach: you can easily get a flat memory-model, but it is not zero-based Quote: Best thing to do for MZ is to allocate another descriptor and make it zero-based, 4 GB in size. Is this need of an additional segment the ONLY problem ? Quote: Then use FS or GS to permanently hold it. For memory transfers with "REP MOVSD" you mightl then have to change DS/ES temporarily. COOL. Seems to work. I can read out the complete DOS memory and write to VGA also, and display a bitmap. Code:
mov cx,1
xor ax,ax
int 31h ; allocate descriptor for data
mov di,ax
mov dx,ds
lar cx,dx
shr cx,8
or cx,0C000h
mov bx,di
int 31h ; set data descriptor access rights
mov ecx,0 ;ZERO
mov edx,0
mov ax,7 ; set descriptor base address CX:DX // CX high
int 31h
mov bx,di ;HACK
int 31h
mov cx,0FFFFh
mov dx,0FFFFh
mov ax,8 ; set segment limit to 4 GB
int 31h ; result : di : data descriptor
Is this ^^^ the correct way to create the segment ? Seems to work. But: 2 segments (start segment from USEDPMI example and my low segment) share same memory: no problem in PM ??? Code:
push ds
pop es ; legal ???
cld
mov ecx,$40000 ; 256K * 4 bytes
mov esi,0
mov edi,$1000
rep movs dword [es:edi],[fs:esi] ; low segment is in fs
Correct way ^^^ to read out all 1 MB, suppposing DPMI segment is 4 MB and code <4 KB ??? Code:
push fs ;low segment in fs
pop es ; legal ???
cld
mov ecx,16000 ; 320*200 = 64000
mov esi,$3F0600 ;stored in last 64'000 bytes
mov edi,$A0000
rep movs dword [es:edi],[ds:esi]
Writing ^^^ complete 64'000 bytes into VGA - correct ? Seems to work. _________________ Bug Nr.: 12345 Title: Hello World program compiles to 100 KB !!! Status: Closed: NOT a Bug |
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31 Dec 2006, 03:36 |
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