USR

Allows the usage of a ROM- or user-implemented subroutine in machine language

Implemented by: Altair, Applesoft, CBM, Atari, msatari, level2, trsdisk, level3, trscolor, Sinclair, BBC, GWBASIC, MSX

With variations: altair12K (uses H, L registers instead of D, E)

Also written as: