IOT Entry, Decoding and Dispatch

The Iinput/Output Transfer instruction handler is written in FORTRAN as a zero argument subroutine.

SUBROUTINE PDP8IOT

Upon entry the routine decodes the IOT instruction which has been left in the (now stored) memory data register. Two levels of dispatching take place. The first dispatch is to device handlers based on the instruction's device id (ID). A second level dispatch in most of these handlers decode the meaning of the IOP pulses in the context of the selected device.

      ID=(ISHIFT(MD,-3).AND.77B)+1
      IO=(MD.AND.7B)+1
      IF(ID.LE.5) GOTO (200,210,220,230,240),ID
      IF(MD.EQ.6214B) GOTO 302
      IF(MD.EQ.6201B) GOTO 106
      GOTO 404

Program data transfer:

200 CONTINUE
GOTO 404

Paper tape reader:

210 GOTO (404,108,212,404,106,404,212,404),IO

Paper tape punch:

220 GOTO (404,108,106,404,224,404,224,404),IO

Teleprinter keyboard:

230 IF(MD.EQ.6032B) GOTO 106
GOTO 404

Teleprinter printer:

240 GOTO (106,108,106,404,243,108,243,404),IO

PDP-8 Simulator