TSC8-75 Board for ETOS

The TSC8-75 board is a proprietary piece of hardware for the PDP-8/E required to run ETOS, EduComp's Timeshared Operating System (later called Extended Timeshared Operating System when EduComp was renamed to QuoData).

The following description of the functioning of the TSC8-75 board was derived from the TSC8.SV hardware diagnostic program (available on the ETOS disk images; to convert them to the image format required for the PDP-8/E Simulator, use these download links: etosv5b-demo.rk05, etosv5b-pl5-config.rk05, etosv5b-pl5-dist.rk05), from the ETOS manuals (testing the ETOS board is described in the System Manager Guide chapter 2.11, pp. 2-41 – 2-48) and the ETOS white paper. The functioning was verified by implementing a TSC8-75 device for the PDP-8/E Simulator that successfully runs the TSC8.SV hardware diagnostics and ETOS.

The TSC8-75 provides the following registers:

Every TSC8-75 provides the following IOT instructions (at I/O address 36). The mnemonics were retrieved from the error messages of the TSC8.SV diagnostics.

Mnemonic   Opcode   Description  
ETDS 6360 Disable the TSC8-75 by clearing the enable and interrupt flag.
ESKP 6361 Skip the next instruction when the interrupt flag of the TSC8-75 is set.
ECTF 6362 Clear the interrupt flag of the TSC8-75.
ECDF 6363 Move ERIOT(6-8) to AC(9-11) by performing a logical OR (when ERIOT holds a CIF, CDF or CDI, this is the field number of the instruction). When the ECDF flag is set (then ERIOT holds a CDF instruction (62x1)), additionally the next instruction is skipped. The ECDF flag is cleared.
ERTB 6364 Clear AC, then move the ERTB register to AC.
ERIOT 6366 Clear AC, then move the ERIOT register to AC.
ETEN 6367 Enable the TSC8-75 by setting the enable flag.

TSC8-75 boards starting with serial number 699 additionally provide the so called ESME feature (see ETOS System Manager Guide chapter 3.5, pp. 3-10 – 3-11). This feature enables the software to skip the emulation of unnecessary CDF instructions when the data field of the CDF is the current field. The TSC8.SV diagnostics is supposed to test the ESME feature (when SR9=1), but it gives no indicator for what the feature actually does. Inspecting the interrupt service routine of ETOS shows that the ESME feature is implemented by the IOT 6365 (this is the first instruction in the interrupt service routine), refered by the mnemonic ESME (the original mnemonic is unknown):

Mnemonic   Opcode   Description  
ESME 6365 Serial number of TSC8-75 board before 699: NOP
Serial number of TSC8-75 board 699 or higher: Skip the next instruction when the ECDF flag is set (then the ERIOT register contains a CDF opcode (62x1)) and the target data field of this CDF in ERIOT(6-8) is equal to SF(4-6), the data field that was active before the last interrupt. When a skip occurs, the ECDF flag is cleared, otherwise not.

When a TSC8-75 board is installed in a PDP-8/E, the following instructions behave not as usual when executed while the processor is running in user mode: