Device I/O Instructions
(Typically never used by an application programmer, but used extensively in the O/S to control all peripherals such as terminals, printers and disk drives. I suspect they are also used for later functions such as bank switching and the EP's real-time clock.)
I/O devices may have up to three 17-bit Device Registers, accessible to the programmer, implemented by their hardware interface. By convention, Device Register 3 is the Status Register.
All I/O devices also have a BUSY flag and a DONE flag.
Start a device by setting its BUSY flag (after loading relevant device registers). The device signals an INTERRUPT REQUEST whilst its DONE flag is set.
(The MASK instruction can/could be used to limit which devices generate an interrupt. Not sure if this is supported post the Mk 4)
| 17 | 16 | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
| 0 | 0 | 0 | 0 | 1 | 1=A-Reg 0=B-Reg | Function or Skip Condition | Mode | Device Code | ||||||||
| B9:B7 | Mnemonic | Action |
| 1 | DATI1 | Read Device Register 1 |
| 2 | DATI2 | Read Device Register 2 |
| 3 | DATI3 | Read Device Register 3 |
| 4 | DATO1 | Write Device Register 1 |
| 5 | DATO2 | Write Device Register 2 |
| 6 | DATO3 | Write Device Register 3 |
| 7 | SKIP | Test for SKIP CONDITION |
Function or Skip Condition (B11:B10)
| B11:B10 | Function | SKIP IF |
| 0 | No Operation | BUSY |
| 1 | START (Set BUSY, Clear DONE) | NOT BUSY |
| 2 | STOP (Clear BUSY, Clear DONE) | DONE |
| 3 | IOPLS (Input/Output Pulse) | NOT DONE |
Device Codes (B6:B1)
| In | Out | Standard Assignment |
| 20 | 60 | Alpha-numeric Keyboard |
| 50 | 40 | Visual Display Unit |
| 30 | Line Printer | |
| 34 | Serial Printer | |
| 66 | 67 | IBM I/O Writer |
| 11 | 33 | Paper Tape |
| 70 | 70 | Disc |
Example code to read a single sector from disk (actually part of the LOS bootstrap)
(The bootstrap is deliberatly simple for toggling in. It loads one sector starting at sector 0 into core starting at address 0. Interrupts are off on power-up, so the bootstrap sits in a tight loop just after this waiting to be overwritten by a jump to an lower address. The sector is still coming in to core via DMA.)
| 010670 | DATO3B | Load sector No to reg 3 |
| 010570 | DATO2B | Load target core address to reg 2 |
| 011470 | DATO1B SBCD | Load reg 1 and start |
Example code to send a single character to a terminal
Example code to read a single character from a terminal