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Class Extensions

The value 15 in the Class field indicates a "Class Extension". Those are classes that don't
make use of fields OTA, OTD, LD. The circuitry to decode extended class instructions are in
IDS Extended cards.

Up to 15 class extensions can exists. They are indicated in the 4-bits field "Ext" (D9-11):

[15][Ext][arg]

The field "arg" is 8-bits. Its usage is determined by the Extension Class. At the time of writing, the following Class Extensions have been defined:

[15][1][vector]

(Vector is 8-bits)

Difference between RET and RETI is that the latest activates the ISE (Instruction Service End) control line in the External Bus as par of the negotiation with the interrupter peripheral (See Interrupt Architecture).

[15][2][0]         RET
[15][3][0]         RETI