The Aleph event builder: a multi-user FASTBUS master

A description is given of the Aleph event builder, a 68020-based FASTBUS master with a special coprocessor executing FASTBUS transactions as single assembly instructions. Keeping the original architecture, the hardware and firmware have recently been upgraded to implement the standard FASTBUS calls. Full multiuser, multitasking support is provided to handle up to 16 concurrent environments without the need of any software driver. Several application programs can execute FASTBUS calls without intermediate software levels, each one having a virtual FASTBUS controller at its disposal. Environments are fully independent, both for execution of normal instructions and exception-handling. No save/restore operation is necessary and the overhead for context switching is reduced to zero because all multiuser aspects are controlled directly in hardware at the coprocessor level. The performance for multiuser FASTBUS access has been greatly enhanced both in speed and simplicity.<>