I/O - BCL Molecular 18

BCL Molecular 18
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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)

1716151413121110987654321
000011=A-Reg
0=B-Reg
Function or
Skip Condition
ModeDevice Code


B9:B7MnemonicAction
1DATI1Read Device Register 1
2DATI2Read Device Register 2
3DATI3Read Device Register 3
4DATO1Write Device Register 1
5DATO2Write Device Register 2
6DATO3Write Device Register 3
7SKIPTest for SKIP CONDITION

Function or Skip Condition (B11:B10)


B11:B10FunctionSKIP IF
0No OperationBUSY
1START (Set BUSY, Clear DONE)NOT BUSY
2STOP (Clear BUSY, Clear DONE)DONE
3IOPLS (Input/Output Pulse)NOT DONE

Device Codes (B6:B1)

InOutStandard Assignment
2060Alpha-numeric Keyboard
5040Visual Display Unit
 30Line Printer
 34Serial Printer
6667IBM I/O Writer
1133Paper Tape
7070Disc

Example code to read a single sector from disk (actually part of the LOS bootstrap)

(The bootstrap is deliberately 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.)


004400CLEAR B
010670DATO3BLoad sector No to reg 3
010570DATO2BLoad target core address to reg 2
011470DATO1B SBCDLoad reg 1 and start


The OS bootstrap

The OS bootstrap is a little more complex as it supports both Hawk and Lark drives, but the principle is the same.  Rather than simply loading sector 0 from the drive, it loads sector 200.  Obviously this can be changed and it make one wonder what other sectors could be booted instead?

100010670DATO3B
101011470DATO1B SBCD
102210107LDA 107Load A with boot sector number
103015570
104012770
105020104JUMP 104
106020200JUMP 200Jump into the boot sector
107000200Sector No 200

For DD1600 (Hawk) as Unit 70:
Reg A all zeros
Reg B Bit 17: 1 for fixed or 0 for exchangeable
Reg B Bits 16-15: drive code (0 to 3)
Reg B Bits 14-1: MUST be zero

For DD9600 (Lark) as Unit 70:
Reg A set to 100(8) i.e. Bit 7
Reg B Bit 17-16: drive code (0 to 3)
Reg B Bits 15-11: surface code (0 to 3)
Reg B Bits 10-1: all zeros

Example code to send a single character to a terminal

Example code to read a single character from a terminal




(C) 2022 Kevin Murrell & The National Museum of Computing
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