| 1 339 | 0 968 | |
| (34.01) | (24.58) - i- PIN l INDICATOR | 0.147 (3 t3) |
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| | f |
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knowledged. The in-service bit of a particular chan- nel may be cleared by writing a zero to the corre- sponding bit in ISRA or ISRB. Typically, this will be done at the conclusion of the interrupt routine just before the return. Thus no lower prionty channel will be allowed to request service until the higher priority channel is complete, while channels of still higher priority will be allowed to request service. While the in-service bit is set, a second interrupt on that chan- nel may be received and latched into the pending bit, though no service request will be madein re- sponse to the second interrupt until the in-service bit is cleared. ISRA and ISRB may be read at any time. Only a zero may be written into any bit of ISRA and ISRB ; thus the in-service bits may be cleared in soft- ware but cannot be set in software. This allows any one bit to be cleared, without altenng any other bits, simply by writing all ones except for the bit position to be cleared to ISRA or ISRB, as with IPRA and
