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Support for the Console START button

When the Operator presses the START button in the Console, an NMI interrupt request is asserted to the microprocessor which service routine is at address 0066H. This address contains a jump to the START routine listed below:

START
    in    a, (SARL)
    ld    l, a
    in    a, (SARH)
    ld    h, a
    jp    (hl)

SAR is the Start Address Register (16 bits, split in two 8-bits registers SARL, SARH) located in the Console Register Board in the MC Unit. This register is loaded from the Console's swiches when the LOAD button is pressed, then loaded into the Program Counter register (PC) by this routine (as we can infer from the code) when the START button is pressed.

These seven bytes of machine code is the only software support that the Console requires. The rest of its functionality is based on hard-wired logic.

LC-81 Homebrew Minicomputer -- this software is based on Help Books running at melissa