Tools
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Assembler:
NASM - Netwide Assembler
http://www.web-sites.co.uk/nasm/index.html
ftp://ftp.simtel.net/pub/simtelnet/msdos/asmutl/

Linker:
ALINK - Linker
http://alink.home.dhs.org
which redirects to
http://www.geocities.com/SiliconValley/Network/4311/