norman@cs.purdue.EDU (Victor Todd Norman) (01/18/90)
Question: Is there a way for a program to dynamically figure out how large the memory space is? I have tried to figure out this value by having my program write a value to memory and then read it back, checking if the value is the same. I start doing this somewhere that I know exists and try to walk off the end of memory. I was hoping that as soon as I fell off the end, the value would be changed. But, this doesn't work since, as I've found out, memory seems to wrap around. Example: I am using a T414 (2K on-chip) with 2M external memory. Thus, the highest address is #80200800. But, when I write a value to somewhere off in never-never land, like #802037be, I can read the value back from that location. Or, I can find the value in #800037be (which is where it actually exists all along). So, my idea of checking legal addresses by writing to them doesn't seem to work too well. Any suggestions? Victor Norman
rohran@ADAM.BYU.EDU (Richard Ohran) (01/18/90)
Here is a procedure written in Modula-2 which finds all the memory between the top of the process and the top of memory. It returns the starting address in the variable "adr" and the number of available bytes in "size". It checks for both memory wraparound and non-existent memory address regions. PROCEDURE Allocate(VAR adr: POINTER TO BITSET; VAR size: CARDINAL); VAR x: POINTER TO ProcessState; z,zchanged:BITSET; b:POINTER TO BITSET; BEGIN (*size is in bytes*) InitProcessPtr(x); adr:=x^.ProcessTop; (* adr now has the first memory address above the running program*) size:=0; LOOP INC(adr,200H); b:=CARDINAL(adr) MOD 10000H + 80000000H; (*creating wraparound address*) IF adr^=b^ THEN (*potential memory wraparound*) z:=adr^; zchanged:=z/{0..31}; (*exclusive or*) adr^:=zchanged; IF adr^=b^ THEN (*wraparound*) adr^:=z; EXIT END END; (*now check for non-existent memory*) z:=adr^; z:=z/{0..31}; (* exclusive or*) adr^:=z; IF adr^<>z THEN EXIT END; adr^:= z/{0..31};(*restore data for valid word*) INC(size,200H); END; adr:=x^.ProcessTop; END Allocate; Hope this helps. If you have any questions about how it works, call me at 801-226-8978. Richard Ohran
EDMONDS@prlvax1.prl.philips.co.uk (Mark from Philips) (01/18/90)
You can determine memory size dynamically by using the variable 'freespace'. freespace gives you the total amount of memory left over after your program (and other programs, e.g. compiler on a host) has been loaded. Since occam has no dynamic features, freespace is a constant. You can even use it, e.g. as an array: VAL freesize IS freespace : []INT array IS [freespace FROM 0 FOR freesize] : However, you do need to know what other programs are currently loaded into memory and the size of their program and data spaces - but that shouldn't be too tricky to work out. In fact, you could even write a process (a memory manager) that allocates freespace to itself, and accepts requests from other processes for bits of it. That way, you can (sort of) have some sort of dynamic memory arrangement (procedural abstraction?). Mark Edmonds Philips Research Labs edmonds@uk.co.philips.prl