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Hidden registers

Hidden registers are those not accessible to the programmer in the sense that there no instructions to read or write them directly.

G is a name given to a collection of unrelated individual flip-flops:

   FF     Meaning when set
   ---------------------------------
   IE     interrupt is enabled
   TE     fail trap is enabled
   Fail   fault condition detected
   ----------------------------------

This register is cleared on reset, resulting in a halted machine with no pending conditions.

The LAR register latches the word being displayed in the Console ADDR display. Actually, those lamps are fed from LAR's outputs.

When the machine is halted, LAR provides the address to actually address the memory in operations such as examining and altering memory content.

During normal program execution, LAR is updated (latched) for each machine cycle.

Similar to LAR, the Lamps Data Register (LDR) holds the DATA display reading.

LDR register is fed from the Lamps Bus (L-BUS) which is fed in turn from the selected register or memory (M) as the operator selects so in the Console.

When the machine is in normal program execution, if M is selected, LDR is updated for each memory read or write cycle. If a register is selected instead, LDR remains open passing to the Display the L-BUS content (that is, the selected register).

When the machine is halted, LDR is the destination of memory reading cycles made by the Console circuitry when the Operator examines memory content.

The BPR register stores a break point set by the Operator.

When the BP/M switch is in its up position, BPR output feeds the ADDR display (instead of LAR). In this condition, BPR will latch the K register content (Entry Switches) if the SET button is pressed, being this the way for the Operator to set the Break Point in the machine.