Programming the 9750 interface in 6502 Assembly

Caution: Do not try this unless you know what you're doing with 6502 assembly code! The following is quoted directly from pages 4.16-4.17 of the Lego Lines manual. I am not providing advice in this language.

Although the Lego interface can be programmed in BASIC, some of the Lego Lines program is written in 6502 machine code, to improve performance.

All hexadecimal values are denoted by the prefix $ (for example, $2A), while binary values are denoted with the prefix % (for example, %101100).

The address of the interface I/O port is given by the following:

LEGO EQU $C080+$10*S

where S is the slot number of the Lego interface. Normally, where the Lego interface is in slot 2, the address is given by:

LEGO EQU $C0A0

The correct initialization sequence for the interface is as follows:

All data is written to address $C0A0. The six data bits on the interface correspond exactly to the six data least significant bits written to the address. So, for example, to turn on bits 1 and 3 only (and the rest off), it is necessary to load the accumulator with binary %001010 ($0A).

Below is the routine to send the bits in the accumulator to the interface:

Reading the data is more complex. First, the data must be read from the interface. Then the correct bits must be filtered out, if necessary. Finally, the appropriate bits must be examined.

To read the current status of the output bits (to read which bits are currently on):

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