Microport and SCO Xenix Memory Managment Problems

Tim Smith tim at ism780c.UUCP
Sat Oct 25 14:41:05 AEST 1986

sbrk has to behave funny because of program that assume a linear
address space.  Many program that want N bytes do the following:

		char * ptr;
		ptr = sbrk(0);

To make these work correctly, sbrk(0) has to return the start of
the next segment.

For example, here is how brk and sbrk work on the AT&T pc6300+ (
which is a good indication of how AT&T thinks things should work
on 80286 based machines )

	( all byte counts and addresses are rounded to a multiple
	  of 512 )

	In small model, everything looks like a normal unix system

	In large model,

		sbrk(0) returns start of next data segment

		sbrk(N) N > 0 allocates N bytes from next data segment

		sbrk(N) N < 0 does this:
			N = -N
			while ( N >= size of last segment )
				N = N - size of last segment
				remove last segment
			remove N bytes from last segment

		brk(addr) ( where addr == byte N of segment S )
			if addr in current last segment, then set
			segment size to N

			if addr in next segment, then same as sbrk(N)

			if addr in an allocated part of a previous segment
			then throw away everything above addr

			else  error
member, all HASA divisions              POELOD  ECBOMB
                                               ^-- Secret Satanic Message

Tim Smith       USENET: sdcrdcf!ism780c!tim   Compuserve: 72257,3706
                Delphi or GEnie: mnementh

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