The Flags Register (F)The Flags register (F) resides in the Master Controller card (MC) which plugs to both Data and Control slots. The register is 8-bits but only 4 are used to remember the Conditional Status; a 5th. bit is used to remember the Interrupt Enabled condition as we shall see; the remaining 3 bits are implemented but not used (reserved for future use). The whole register F is saved to the Stack during CALL or Interrupt and restored back upon Return.
The register is actually made out of D-type flip-flops (74ACT74) fed from the S-BUS. The outputs go to another bus (called CS-BUS) wired in the Control Slots; this bus is always open so the current status is always available to Controller cards. Flags are latched selectively using signals LD_FZ, LD_FN, LD_FC and/or LD_FV produced by IDS cards during the last cycle of those instructions that affect flags.
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