PDP-8 Processor Core and System

I compiled the minimal version into XC6SLX9 (ISE 14.7). After the reset I get garbage chars when typing enter (or any text), after typing "DIR" I get the listing visible.

® ® ® ®ÄÉ? << DIR

DIRECT.SV 7 FRTS .SV 26 BASIC .WS 11 PIP .SV 11 PASS2 .SV 20 SET .SV 20 FOTP .SV 8 PASS2O.SV 5 RTFLOP.SV 15 HEADER.SV 14 PASS3 .SV 8 WPFLOP.SV 14 BITMAP.SV 5 RALF .SV 19 RESORC.SV 14 FUTIL .SV 27 LOAD .SV 16 DECX8 .SV 43 SVDUMP.SV 4 MOVE .RA 4 BUILD .SV 37 REMEM .SV 5 PUT .RA 3 KL8E .PA 53 ... K12ENB.SV 4 FLOP .HN 3 ATEXT .DA 144 BRTS .SV 15 LCSYS .BI 3 ADVENT.LD 145 F4 .SV 20 UCSYS .BI 3 LOAD .LS 8

162 Files in 2648 Blocks - 544 Free blocks

®ÔÈÉÓÉÓÁÔÅÓÔ® << THISISATEST

Note: the directory listing above is properly formatted..

Congratulations Pito! There is absolutely nothing wrong with your PDP8 FPGA. This is exactly what a real PDP8 would display using your terminal settings! Welcome to being a retro fan...

It surprised me and surprised others. Maybe this should be a FAQ?

The PDP8 character set was designed when ASCII was a new standard. The PDP8 software assumes that the output device is a real teletype machine which ignores the MSB of the data. No modern device does that. In some places, the PDP8 software will set the MSB of the output data and you will see gibberish. In other places (directory command, for example) it is correctly done.

You can work around that by configuring your terminal settings to 9600 bps, 7-bit data, mark parity, and 2 stop bits. Not all terminal emulators will do mark parity - TeraTerm and PuTTY will.

That is 9k6 7m1

Alternately 7-bit data, no parity, and 2 stop bits usually works.

The PDP8 was never 8-bit clean on ASCII.