weird "gaps" in process's memory map

two questions, actually.

(1) pmap shows me this for my shell (tcsh):

is# pmap -lv
00008000-0005b000 r-xp 00000000 10:04 682285     /usr/pkg/bin/tcsh
00060000-00065000 rw-p 00050000 10:04 682285     /usr/pkg/bin/tcsh
00065000-00092000 rw-p 00000000 00:00 0       [ anon ]
00092000-000e2000 rwxp 00000000 00:00 0       [ heap ]
20060000-20069000 r-xp 00000000 10:00 9383     /libexec/ld.elf_so
20070000-2007a000 rw-p 00000000 00:00 0       [ anon ]
2007a000-2007d000 r-xp 00000000 10:00 23     /lib/libtermcap.so.0.5
2007d000-20084000 ---p 00003000 10:00 23     /lib/libtermcap.so.0.5
20084000-20085000 rw-p 00002000 10:00 23     /lib/libtermcap.so.0.5
20085000-2008b000 r-xp 00000000 10:00 15     /lib/libcrypt.so.0.1
2008b000-20092000 ---p 00006000 10:00 15     /lib/libcrypt.so.0.1
20092000-20093000 rw-p 00005000 10:00 15     /lib/libcrypt.so.0.1
20093000-20096000 rw-p 00000000 00:00 0       [ anon ]
20096000-2013c000 r-xp 00000000 10:00 13     /lib/libc.so.12.91
2013c000-20143000 ---p 000a6000 10:00 13     /lib/libc.so.12.91
20143000-20149000 rw-p 000a5000 10:00 13     /lib/libc.so.12.91
20149000-20157000 rw-p 00000000 00:00 0       [ anon ]
ef3fe000-ef9fe000 ---p 00000000 00:00 0       [ stack ]
ef9fe000-efbe0000 rwxp 00000000 00:00 0       [ stack ]
efbe0000-efbf0000 rwxp 00000000 00:00 0       [ stack ]
efbf0000-efbfe000 rwxp 00000000 00:00 0       [ stack ]

and it stuck me that there are chunks of shared libraries mapped in
with no read, write, or execute permission.  can anyone tell me why?
i'm curious...

(2) i note the half gigabyte gap which is MAXDSIZ between the start of
the data segment and the dynamic linker.  why was one half gigabyte
chosen?  if we could, would it be better to map the dynamic linker
(and shared libraries) in directly beneath the stack?

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