| 摘要 |
PURPOSE:To test an instruction decoder part with no production of a test program by adding a test circuit to the instruction decoder to send directly the working state of the instruction decoder to an outside. CONSTITUTION:The signal lines 6a-6c show the 1st-3rd bit signal lines of an instruction code respectively. While the control signal lines 7a-7h select the instruction codes (000)2, (011)2, (101)2, (110)2, (001)2, (010)2, (100)2 and (111)2 respectively. An instruction decoder 3 selects just one control signal to an instruction code. When the instruction code shows (000)2, for example, only the line 7a outputs 0 and other signal lines output 1. At the same time, an external output terminal 9 outputs 1. While both lines 7a and 7e are selected at one time and output 0 by the short-circuit of a transistor within the decoder 3. Thus the terminal 9 outputs 0 to inform an error to the outside. |
