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)

Function or
Skip Condition
ModeDevice Code

1DATI1Read Device Register 1
2DATI2Read Device Register 2
3DATI3Read Device Register 3
4DATO1Write Device Register 1
5DATO2Write Device Register 2
6DATO3Write Device Register 3

Function or Skip Condition (B11:B10)

B11:B10FunctionSKIP IF
0No OperationBUSY
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

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.
Note that the disk controller always loads one more sector than specified in the control word, so in the case we can leave the sector count at zero, and it will still load one sector.

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?

101011470DATO1B SBCD
102210107LDA 107Load A with boot sector number
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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