Each process gets a single segment for data+stack+heap. This is necessary
because 'C' pointers must be usable interchangeably between objects in these
three areas. The bcc compiler currently assumes SS=DS=ES.

+------------+--------------------------------------------------+
| data + bss |  heap ---> |                          <--- stack |
+------------+--------------------------------------------------+
                          ^                                     ^
                  current->t_enddata                  current->t_endstack

Memory allocation for a new process is done in sys_execve() in fs/exec.c.
The binary file header, defined in <linuxmt/minix.h>, gives the sizes of the
areas to allocate:

  tseg  (bytes 8..11)    - code size
  dseg  (bytes 12..15)   - data size
  bseg  (bytes 16..19)   - bss size [uninitialised data, filled with zeros]
  chmem (bytes 24..27)   - total data segment

Hence current->t_enddata is initialised to dseg+bseg, and
current->t_endstack to chmem. The position of current->t_enddata can be
changed by the "sys_brk" system call, which is in turn used by the malloc
functions in libc.

When the linker (ld) creates a binary it gives a default setting for chmem
(see writebin.c in the ld source). If you know that a particular program
uses little or no heap, you can patch bytes 24..27 in the executable to a
lower value: dseg+bseg+desired stack size.