EDUC-8L: Adding a Link Bit to the EDUC–8 Microcomputer

[Steven Pietrobon]

I have finally completed the design and documentation of EDUC-8L, which adds a link bit to EDUC-8 along with 16 new instructions. The modifications add a total of 12 additional chips to EDUC-8, or 0.75 chips per instruction! You can download my paper on EDUC-8L using the link below. The paper includes an updated Program Encoding Guide as well as five pages of schematics.

http://www.sworld.com.au/steven/educ-8/educ-8l.pdf

Next up is adding interrupts to EDUC-8. When writing to the input device (ID) or output device (OD) field, the data in the AC before writing will be XYZ, in 737 format. X selects one of eight EDUC-8ME units. Z selects one of the eight ID or OD ports. Y is normally 0. Each unit will have two 74259 addressable latches that contains the interrupt enable for each ID and OD port. If Y = 2, then the interrupt enable for port Z is disabled. If Y = 3 the interrupt enable for port Z is enabled. If Y = 1, then all the interrupt enables are disabled.

Similar to PDP-8, the additional instructions are

605 IOF Interrupts Off
615 ION Interrupts On (after one additional instruction)
625 SON Skip if Interrupt System On
635 SRQ Skip on Interrupt Request

There are no special instructions to get all the registers, like with PDP-8, since AC does not have enough space to do that. The AC, L, IF, DF, ID and OD will need to be saved using existing instructions. On an interrupt (which occurs if a device flag goes high and its interrupt enable latch is on), the IF will be loaded into the EDUC-8ME parallel to serial converter, after which the IF will be cleared.

The PC will be loaded into address 010 (to avoid using the eight page zero values) with instructions starting at 011. After saving AC and L, the first read from EDUC-8ME must be of the IF, as the interrupt state machine will disable loading the parallel to serial converter to prevent the IFR instruction reading the actual all zero IF. It will instead read the IF before the interrupt. After the first read from EDUC-8ME, the DF, ID and OD can then be normally read and saved