There is a GNAT ada compiler that can be used for ARM. For example, there is a debian (linux) GNAT package for ARMel: http://packages.debian.org/squeeze/gnat
There is even a version of GNAT from an ARM AdaCore partner called GNAT Pro: http://www.arm.com/community/partners/display_product/rw/ProductId/5620/
IBM Rational Ada Embedded only works for MIPS, M68K, x86, RH-32, maybe SPARC.
Irvine ICC Ada only works for x86; SPARK; TI DSP C62xx / C64xx / C67xx; PowerPC 5xx, 6xx, 7xx; Intel i960; Intel i960; ADSP-210x0 SHARC DSP.
Green Hills Ada95 only works for: Power, 68k, CPU32, MIPS, x86, SPARC (docs)
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