Issues I found at a glance:

1. This uses unsigned int for the chunk size, so the allocator will overflow on requests of 4GB or more despite taking a size_t. It seems that this is 32-bit only.

2. Even on 32-bit, the num_units calculation will overflow if you request (for example) 0xffffffff bytes of memory instead of returning an error.

3. None of this is thread-safe. It needs a global mutex lock.

4. EBP cannot be relied upon to yield anything sensible with -fomit-frame-pointer, which is common on 32-bit x86 as it brings the number of GPRs from 6 to 7.