mitsubishi - Al Kossow's Bitsavers

MITSUBISHI MICROCOMPUTERSMELPS 8·48 MICROCOMPUTERSFUNCTION OF MELPS 8-48 MICROCOMPUTERSnext instruction and begins to execute it, but then an ALEsignal is sent to the reset terminal of the type D flip-flopwhich turns SS to low-leveL The CPU again stops as soonas execution of the current instruction is completed. Whenthe push-button switch is again pushed, the cycle is repeatedand the CPU is in single-step operation as shown in Fig.12. While the CPU is stopped in single-step operation, thedata bus and the low-order 4 bits of port 2 are used to outputthe memory address of the next instruction to be fetched.This interferes with input and output, but essentialinput/output can be latched by using the rising edge ofALE as clock.Central Processing Unit (CPU)Central Processing Unit (CPU) is composed of an 8-bit parallelarithmetic unit, accumulator, flag flip-flop and instructiondecoder. The 8-bit parallel arithmetic unit has circuitry to perform the four basic arithmetic operations(plus, minus, multiply and divide) as well as logical operationssuch as AND and OR. The flag flip-flop is used toindicate status such as carry and zero. The accumu latorcontains one of the operations and the result is usuallyretained in the accumulator.WHEN SS IS LOW, THE CPU RECOGNIZES THATIT IS TO STOP.~WHEN THE NEXT INSTRUCTION IS FETCHED,THE CPU SETS SWITCHES SO IT WI LL STOP AFTERTHE EXECUTION OF THE INSTRUCTION IS COM-PLETED.~WHEN ALE IS HIGH, THE MEMORY ADDRESS OFTHE NEXT INSTRUCTION TO BE FETCHED IS OUT-PUT THROUGH THE DATA BUS 18 BITS) AND THELOW-ORDER 4 BITS OF PORT 2.~WHEN SS IS RETURN'ED TO HIGH LEVEL THECPU RECOGNIZES THAT IT IS IN THE RUN MODE.THEN THE ALE SIGNAL GOES TO LOW-LEVELWHICH INDICATES THE CPU IS IN THE RUN MODEAND THAT IT IS EXECUTING INSTRUCTIONS.~IF THE CPU IS IN THE SINGLE-STEP MODE IPRE-SET TERMINAL GROUNDED). AS SOON AS ALEGOES TO LOW-LEVEL, SS GOES TO LOW-LEVELISTOP). IF THE CPU IS THE RUN-MODE IPRE-SET TERMINAL NOT GROUNDED), SS WILL RE-MAIN AT HIGH-LEVEL.IIFig. 15 CPU operation in single-step mode• MITSUBISHI"ELECTRIC4-13