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> DOS > How to determine amount of free conventional memory? |
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vid 14 Mar 2004, 13:20
interrupt 12h (? maybe 12d) returns you total amount of conventional memory. Check some interrupt list.
(btw, if it's less than 640kb you probably have some MBR virus (or very old computer)) |
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14 Mar 2004, 13:20 |
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slave17 16 Mar 2004, 12:31
As far as i know int 12h returns the value calculated by BIOS during startup and stored in 40h:13h.the cause for the limit not being 1mb is for example the VGA standard which says that the video memory for 256 color modes (and other modes too) should be mapped begging at 0a0000h which is exactly 640kb.the may also be other devices mapping their registers or memory between 640kb and 1mb.the memory manager himem(or emm386 - frankly i dont remember) somehow decides on its own which part of this memory is not mapped and can safely be used for UMB.UMBs are addressed the same way as base mem so i dont see any good reason for not offering all free mem under 1mb as conventional mem - even if it would be divided into blocks of diffent size(ex. 0 to 09ffffh and 0c0000h to 1mb)
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16 Mar 2004, 12:31 |
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slave17 16 Mar 2004, 12:39
i didnt finish my thought: i dont want to read this val from anywhere,i would like to check this value somehow,and also the umb.i want to make almost 1mb base mem available to progs,because this makes things easier(join base and umb).in such situation i cant just read out values,because they are relevant only for dos standard solution.
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16 Mar 2004, 12:39 |
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vid 16 Mar 2004, 19:18
i remember there was somthing about this (not exactly, but may help) in OS construction part.
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16 Mar 2004, 19:18 |
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slave17 19 Mar 2004, 13:53
thanx a lot
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19 Mar 2004, 13:53 |
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slave17 05 Apr 2004, 18:14
now i have naother problem: my prog checks for XMS too.the only values i can check by calls are total free XMS and lergest free EMB so i try to add the sizes of the blocks corresponding to all handles (usually 32 but may vary) in order to get used and total XMS.but this way sometimes i get zero used mem while dos' mem shows that some amount is in use.does ao know where my mistake is?checking all 64k possible handles is slow and even hangs the machine so i wrote i function checking the amount of handles by reserving all free handles to zero length blocks then i check with a function call whether handles with higher numbers exist than those that i reserved.in either case i search the highest numbered handle by comparison and assume its number to be the total amount of XMS handles in the system then i walk through all the handles using once more the same call and sum the sizes(i make here the assumption that the handles are subsequent numbers from 0 upwards).if so knows please tell me another method,dos' mem checks direct free mem by call and total mem(sometimes falsely) in an unknown way.My method finds up to 40k handles on my win98.
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05 Apr 2004, 18:14 |
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rugxulo 16 Aug 2005, 01:57
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16 Aug 2005, 01:57 |
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Matrix 16 Aug 2005, 04:44
yo slave17
getting free conventional memory i remember, you check value of ds at program start and sub it from $a000, the video memory framebuffer starts at the end of free memory, btw, com file allocates all free memory for your use unless you free unwanted memory after start. getting total amount of conventional memory should be done via bios for compatibility reasons, but everything can be done via ports - and even more - but note that every motherboard can be different, so your program whould run only on one of them. you cant get 1 MB conventional memory, because: -there is aprox : 384KB reserved for bios - in most cases, -64KB at $a000 -?64KB at $b000 - text mode framebuffer -interrrupt vector table at the beginning -some os code must be in the conventional memory apart from these, you can be able to emulate some conventional memory, if you write or find a driver ( i saw one long time ago named : to736KB.com http://sac-ftp.externet.hu/utilmisc27.html - but it used video memory for conventional memory so it messed the screen. ) , but since there are limits in use of conventional memory, many programs simply assume you cannot have more than 640KB base memory. |
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16 Aug 2005, 04:44 |
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vid 16 Aug 2005, 19:01
you are using bad terms. As far as i know "convetnional memory" is memory below 640kb boundary (00000 to 9FFFF).
Then there is are "reserved for BIOS" from which usually big part isn't used. On some computers there was some memory located so DOS use this one. It wa called UMB - upper memory blocks. Some software (like Soft-Ice) was able to map RAM from above 1Mb to this region thus allowing things like 700kb of memory for DOS etc. It was also used by EMM386 (usually regions E0000-EFFFF) to map above1Mb ram below 1Mb. Then from F0000 to FFFFF is always located BIOS code (usually it's more than just these 64kb but posiiton isn't always the same). Then there was so-called HMA (high memoy area) which was from 10000 to 1FFEF, which was also accesible by DOS, because segment register can hold values above F000, which allows you to acces a little further above 1Mb. But originally on 286 address was only 20bit, so it was truncated and it wraped from 1000000 to 0. And some software was relying on this behavior, so there was created thing called A20 (21st address line, 20 because indexing from 0) which turned this wrapping on/off on newer processors. So if you disable (or enable, i don't remember  ) this A20 you can access also regions 1000000 to 1FFFFEF (FFFF:FFFF). |
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16 Aug 2005, 19:01 |
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ATV 17 Aug 2005, 04:55
Code: use16 org 0x0100 int 0x12 ;BIOS - Get memory size in k bytes mov dx,0x0400 mul dx call ShowLongint mov dx,txtTotal mov ah,0x09 ;DOS - Display string in ds:dx int 0x21 mov ax,[0x0002] ;get top of memory block mov bx,cs sub ax,bx mov dx,0x0010 mul dx call ShowLongint mov dx,txtFree mov ah,0x09 ;DOS - Display string in ds:dx int 0x21 int 0x20 ;Exit to Dos ShowLongint: ;dx high word, ax low word mov cx,7 ;least 7 digits ShLo0: mov si,dx mov bx,10 ;divisor ShLo1: xchg ax,si ;make ax high word, si low word xor dx,dx ;extend high word div bx ;dx = remainder, ax high quotient xchg ax,si ;save high quotient, get low word div bx ;dx = digit (0-9), ax low quotient or dl,"0" ;convert to ASCII digit ShLo2: push dx ;save digit (remainder) on stack inc ch mov dx,si ;copy so not destroyed below or dx,ax ;is full quotient zero? jnz ShLo1 ;no, continue mov dl," " cmp ch,cl ;If not 7 digits, fill with space jb ShLo2 ShLo3: pop dx ;get most significant digit mov ah,0x02 ;DOS - Display character in dl int 0x21 dec ch ;is more digits in stack jnz ShLo3 ;yes, loop back ret txtTotal db " bytes total memory",0x0D,0x0A,"$" txtFree db " bytes free",0x0D,0x0A,"$" |
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17 Aug 2005, 04:55 |
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rugxulo 27 Sep 2005, 22:45
Try this (although it's MASM/TASM syntax, sorry). EDIT: FREERAM.ASM from TSRW31.ZIP
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27 Sep 2005, 22:45 |
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