MS v5 style HMA access function wrong, requested size *must* be free to succeed

[ecm-pushbx]

As I discussed in a dosemu2 issue: dosemu2/dosemu2#1816 (comment)

kernel/kernel/inthndlr.c

Line 1935 in 5de2eb1

if (r.AH == 0x4a)

I don't understand the if (tsize < size) in my last reference. Shouldn't it check for the requested size being larger and fail the allocation then?

Also another comment by me: dosemu2/dosemu2#1816 (comment)

MS-DOS v5 returns an error (DI = -1) if you try to allocate more than there is available space. So it's valid to never query the size, just init BX with the size you want and try to allocate it. It seems both fdpp and FreeDOS get that wrong, certainly FreeDOS allocates the available size and returns a BX < the requested size.

During my tests I also found that FreeDOS seems to keep track of a byte-granular size and offset, whereas MS-DOS v7 deals in paragraphs. MS-DOS v5 also seems to round everything to paragraphs.

Currently the kernel will allocate HMA space unconditionally in 2F.4A02, shrinking the allocation to whatever space is available if the input in BX is too large. Original MS-DOS (v5 and v7 both) will error out and set DI to FFFFh instead if the requested size cannot be satisfied.

The problem of not aligning allocation offsets + sizes on paragraph boundaries is less critical but may also cause incompatibilities with programs expecting aligned addresses.

I will likely prepared a PR to fix this soon.

[ecm-pushbx]

The use of the remaining (unallocated) HMA as buffers is also questionable, but harder to fix. What if a program (legitimately) allocates the entire space still available in the HMA? Then the kernel has no buffers left??

[ecm-pushbx]

The use of the remaining (unallocated) HMA as buffers is also questionable, but harder to fix. What if a program (legitimately) allocates the entire space still available in the HMA? Then the kernel has no buffers left??

It seems that the available space only stores some of the buffers, a number of them are stored below the free pointer and aren't affected when an application allocates HMA space.

[ecm-pushbx]

This is the function AllocateHMASpace:

kernel/kernel/blockio.c

Line 491 in 5de2eb1

void AllocateHMASpace (size_t lowbuffer, size_t highbuffer)

Its only purpose is to free up space used as buffers if the same space is allocated by an application.

[ecm-pushbx]

Here's where the buffers are allocated:

kernel/kernel/config.c

Line 2157 in 5de2eb1

STATIC void config_init_buffers(int wantedbuffers)

Indeed some buffers are in the free space of the HMA and others below it.