int e l 2.5 DESCRIPTION OF COMMANDS COMMAND: LlST@(cr) (VERSION 1.1 ONLY) ACTION TAKEN: The LIST@ command does the same thing as the LIST command except that the output is directed to a user defined output driver. This command assumes that the user has placed an assembly language output routine in external code memory location 403CH. To enable the @ driver routine the user must SET BIT 27H (39D) in the internal memory of the MCS BASIC-52 device. BIT 27H (39D) is BIT 7 of internal memory location 24H (36D). This BIT can be set by the BASIC statement DBY(24H)=DBY(24H).OR. 80H or by a user supplied assembly language routine.
If the user evokes the @ driver routine and this bit is not set, the output will be directed to the console driver. The only reason this BIT must be set to enable the @ driver is that it adds a certain degree of protection from accidentally typing LIST@ when no assembly language routine exist. The philosophy here is that if the user sets the bit, the user provides the driver or else!!! When MCS BASIC-52 calls the user output driver routine at location 403CH, the byte to output is in the accumulator and R5 of register bank 0 (RB0). The user may modify the accumulator (A) and the data pointer (DPTR) in the assembly language output routine, but cannot modify any of the registers in RB0.
This is intended to make it real easy for the user to implement a parallel or serial output driver without having to do a PUSH or a POP. - 17 -