OSBYTE &81

OSBYTE &81 (129) - Read key with time limit

 On entry: XY=16-bit argument
 On exit:  XY=returned character or state
           Cy=when waiting for a character, no character returned

Description

OSBYTE &81 waits for a character from the current input stream, or tests for a keypress on the keyboard, or returns a value indicating the host machine type. The argument passed in XY is a signed 16-bit value. Positive values (&0000-&7FFF) are a timeout in centiseconds to wait for a character. Negative values (&8000-&FFFF) test for keyboard keypresses, machine type, and any extensions. These are known as negative INKEY numbers.

OSBYTE &81 is called by INKEY and INKEY$ in BBC BASIC, and equivalent calls in other languages and applications.

Wait for input character

 On entry: Y<128 - XY=centisecond timeout - INKEY(0) to INKEY(32767)
 On exit:  Cy=0, Y=0,   X=character : character returned
           Cy=1, Y=255, X preserved : timed out
           Cy=1, Y=27   X preserved : Escape key pressed
 The caller can test for either Cy=0 or Y=0 for a valid returned character.
 If Cy=1 or Y<>0 the contents of X should not be relied on.
 
 Example:  .loop
           JSR tick            :\ Do some processing
           LDY #0:LDX #200     :\ timeout=200cs
           LDA #129:JSR OSBYTE :\ Wait for character
           BCS loop            :\ Loop if no key pressed

Note that Y=27 is returned for a pending Escape state regardless of what actual key is the current Escape key. If Escape is returned the pending Escape state is still pending and the caller will need to acknowledge or clear it at some future point.

Scan for a particular key

 On entry: Y=&FF
           X=&80-&FF negative INKEY number - INKEY(-1) to INKEY(-128)
 On exit:  XY=&0000 - key not pressed
           XY=&FFFF - key pressed

Scan for a range of keys

 On entry: Y=&FF
           X=&01-&7F negative INKEY number - INKEY(-129) to INKEY(-255)
 On exit:  XY=&00FF - key not pressed
           XY=&00nn - key pressed, returns key number
 
 This call scans for a key pressed starting at the passed key number. It is
 rarely implemented.

Check machine type

Depending on what the program needs to know, OSBYTE &00 is usually the more appropriate call to make.

 On entry: Y=&FF
           X=&00
 On entry: X=A value indentifying the subtype of the host (I/O) machine type:
           X=&00 BBC A/B with OS 0.10
           X=&00 RM Nimus
           X=&01 Acorn Electron OS
           X=&FF BBC Micro OS 1.00/1.20/1.23, Reuters OS R0.3 with *UK
           X=&FE BBC Micro (American OS A1.0), Reuters OS R0.3 with *US
           X=&FE NetBSD
           X=&FD Master 128 MOS 3.20/3.50
           X=&FC BBC Micro (West German MOS) - see note
           X=&FC Windows 32
           X=&FB BBC B+ 64/128 (OS 2.00)
           X=&FB Beos (Brandy Basic)
           X=&FA Acorn Business Computer OS 1.00/2.00
           X=&FA Cambridge Workstation OS 2.10
           X=&FA DOS with DJGPP extender (Brandy Basic)
           X=&F9 Communicator
           X=&F9 Linux
           X=&F8 MacOS X (Brandy Basic)
           X=&F7 Master ET MOS 4
           X=&F7 FreeBSD
           X=&F6 OpenBSD
           X=&F5 Master Compact MOS 5.00/5.10
           X=&F5 Amiga (Brandy Basic)
           X=&F4 Master 128 MOS 3.26
           X=&F4 Master Compact MOS 5.11i
           X=&F4 GNU FreeBSD (Brandy Basic)
           X=&F3 GNU (Brandy Basic)
           X=&Ex Spectrum
             X=&E0 ZX Spectrum
                   (INKEY-256 AND &F0)=&E0 gives TRUE for ZX Spectrum hardware
           X=&Dx Amstrad 464/664/6128, etc.
                   (INKEY-256 AND &F0)=&D0 gives TRUE for Amstrad CPC hardware
           X=&Cx Commodore 64/64+/128, etc.
                   (INKEY-256 AND &F0)=&C0 gives TRUE for Commodore hardware
           X=&Bx PDP11 Unix
             X=&B7 PDP11 Unix Version 7
             X=&B6 PDP11 Unix Version 6
             X=&B5 PDP11 Unix Version 5
                   (INKEY-256 AND &F0)=&B0 gives TRUE for PDP11 hardware
           X=&Ax ARM-based systems
             X=&A0 Arthur 1.20
             X=&A1 RISC OS 2.00
             X=&A2 RISC OS 2.01
             X=&A3 RISC OS 3.0x
             X=&A4 RISC OS 3.1x
             X=&A5 RISC OS 3.50
             X=&A6 RISC OS 3.60
             X=&A7 RISC OS 3.7x
             X=&A8 RISC OS 4.0x
             X=&A9 RISC OS 4.3x
             X=&AA RISC OS 5.xx
             X=&AE RISC OS Pyromaniac
             X=&AF Springboard - an ARM-based development system plugged into
                   a PC)
                   (INKEY-256 AND &F0)=&A0 gives TRUE for ARM hardware
           X=&8x Texas Instruments Calculator
             X=&83 TI-83+(SE)
             X=&84 TI-84+(SE)
                   (INKEY-256 AND &F0)=&80 gives TRUE for TI Calculator
                   hardware
           X=&68 6809 system
           X=&63 6309 system
           X=&73 BBC BASIC on SDL abstraction from C source (ie ASC"s")
           X=&57 BBC BASIC for Windows (ie ASC"W")
           X=&53 BBC BASIC SDL abstraction from assembly language source
                 (ie ASC"S")
                 (INKEY-256 AND &DB)=&53 gives TRUE for R.T.Russell's
                 BBC BASIC for Windows/SDL/etc
           X=&50 Sega Master System (ie ASC"P")
           X=&4D Matrix Brandy BASIC (ie ASC"M") for all builds from v1.22.6
                 except RISC OS, and including RISC OS from v1.22.12.
           X=&2A Apple IIgs
           X=&2C Apple IIc
           X=&2E Apple IIe
 
 The known examples of German BBC MOS return X=&FF.

Extensions

The parameter to OSBYTE &81 is a 16-bit number. The standard implementation interprets that as:

 <&8000   - wait for keypress
 &FF00+nn - scan for BBC keypress
 &FF00    - return host machine type

A parameter of &8000-&FEFF is undefined. Some systems extend OSBYTE &81 or INKEY as follows:

 &FE00+nn - scan for low level DOS/Window VK_xxxx keypress
            (cZ80Tube, console library)
 &FC00+nn - scan for low level SDL1.2 SDK_xxx keypress
            (Brandy Basic SDL builds)
 &8000+nn - wait for keypress, return 16-bit character code
            (cZ80Tube, console library)

See Also

• INKEY
• OSBYTE &00
• What BASIC is running
• Negative INKEY numbers
• Keyboard
• http://mdfs.net/Docs/Comp/BBC/Osbyte80
• http://mdfs.net/Docs/Comp/Acorn/HostVals
• http://mdfs.net/Docs/Comp/Acorn/Hosts

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Jgharston 21:56, 26 May 2009 (UTC) Jgharston (talk) 05:20, 23 June 2018 (CEST) Jgharston (talk) 22:09, 25 August 2019 (CEST) Jgharston (talk) 20:03, 16 February 2020 (CET) Jgharston (talk) 07:08, 12 April 2020 (CEST) Jgharston (talk) 00:46, 19 May 2021 (CEST)