Catalyzes the NADPH-dependent formation of L-aspartate-semialdehyde (L-ASA) by the reductive dephosphorylation of L-aspartyl-4-phosphate. L-aspartate 4-semialdehyde + NADP(+) + phosphate = 4-phospho-L-aspartate + H(+) + NADPH Amino-acid biosynthesis; L-lysine biosynthesis via DAP pathway; (S)-tetrahydrodipicolinate from L-aspartate: step 2/4. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homoserine from L-aspartate: step 2/3. Amino-acid biosynthesis; L-threonine biosynthesis; L-threonine from L-aspartate: step 2/5. Homodimer. Cells lacking this gene have an obligate requirement for diaminopimelic acid (DAP) and undergo lysis in environments deprived of DAP. Vaccine strains for Salmonella typhimurium were successfully produced using strains with mutations in the asd gene. Belongs to the aspartate-semialdehyde dehydrogenase family. Extended N-terminus.