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Elite on the BBC Micro

Version analysis of RDKEY

This code appears in the following versions (click to see it in the source code):

Code variations between these versions are shown below.

Name: RDKEY Type: Subroutine Category: Keyboard

Code variation 1 of 7A variation in the comments only

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Cassette, Flight, Docked, Master, Electron

6502SP

Summary: Scan the keyboard for key presses
Summary: Scan the keyboard for key presses by sending an OSWORD 240 command to the I/O processor

Code variation 2 of 7A variation in the comments only

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Cassette, Flight, Docked

Master

6502SP

Electron

Scan the keyboard, starting with internal key number 16 ("Q") and working through the set of internal key numbers (see p.142 of the Advanced User Guide for a list of internal key numbers).
Scan the keyboard, starting with internal key number 16 ("Q") and working through the set of internal key numbers, returning the resulting key press in ASCII.
This routine sends an OSWORD 240 command to the I/O processor to ask it to scan the keyboard, starting with internal key number 16 ("Q") and working through the set of internal key numbers (see p.142 of the Advanced User Guide for a list of internal key numbers). The results are copied from the I/O processor into the key logger buffer at KTRAN.
Scan the keyboard, starting with internal key number 16 ("Q") and working through the set of internal key numbers (see p.40 of the Electron Advanced User Guide for a list of internal key numbers).
This routine is effectively the same as OSBYTE 122, though the OSBYTE call preserves A, unlike this routine. Returns:

Code variation 3 of 7A variation in the comments only

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Cassette, Flight, Docked, 6502SP, Electron

Master

X If a key is being pressed, X contains the internal key number, otherwise it contains 0
X If a key is being pressed, X contains the ASCII code of the key pressed, otherwise it contains 0
A Contains the same as X

Code variation 4 of 7A variation in the comments only

This variation is blank in the Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron versions.

Master

Y Y is preserved Other entry points: RDKEY-1 Only scan the keyboard for valid BCD key numbers

Code variation 5 of 7Specific to an individual platform

This variation is blank in the Cassette, Disc (flight), Disc (docked), 6502 Second Processor and Electron versions.

Master

SED \ Set the D flag to enter decimal mode. Because \ internal key numbers are all valid BCD (Binary Coded \ Decimal) numbers, setting this flag ensures we only \ loop through valid key numbers
.RDKEY

Code variation 6 of 7Specific to an individual platform

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Cassette, Flight, Docked, Electron

Master

6502SP

LDX #16 \ Start the scan with internal key number 16 ("Q") .Rd1 JSR DKS4 \ Scan the keyboard to see if the key in X is currently \ being pressed, returning the result in A and X BMI Rd2 \ Jump to Rd2 if this key is being pressed (in which \ case DKS4 will have returned the key number with bit \ 7 set, which is negative) INX \ Increment the key number, which was unchanged by the \ above call to DKS4 BPL Rd1 \ Loop back to test the next key, ending the loop when \ X is negative (i.e. 128) TXA \ If we get here, nothing is being pressed, so copy X \ into A so that X = A = 128 = %10000000 .Rd2 EOR #%10000000 \ EOR A with #%10000000 to flip bit 7, so A now contains \ 0 if no key has been pressed, or the internal key \ number if a key has been pressed
TYA \ Store Y on the stack so we can retrieve it later PHA JSR RDKEY2 \ Call RDKEY2 to scan the keyboard, update the key \ logger and return any non-logger key presses in X PLA \ Retrieve the value of Y we stored above TAY LDA TRANTABLE,X \ Fetch the internal key number for the key pressed STA KL \ Store the key pressed in KL TAX \ Copy the key value into X
LDA #240 \ Set A in preparation for sending an OSWORD 240 command LDY #HI(buf) \ Set (Y X) to point to the parameter block at buf LDX #LO(buf) JSR OSWORD \ Send an OSWORD 240 command to the I/O processor to \ scan the keyboard and joysticks, and populate the key \ logger buffer in KTRAN, which is the part of the buf \ buffer just after the two OSWORD size bytes LDX KTRAN \ Set X to the first byte of the updated KTRAN, which \ contains the internal key number of the key being \ pressed, or 0 if there is no key press TXA \ Copy X into A

Code variation 7 of 7Specific to an individual platform

This variation is blank in the 6502 Second Processor and Master versions.

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Cassette, Flight, Docked

Electron

TAX \ Copy A into X
TAY \ Store A in Y so we can preserve it through the call to \ CAPSL below JSR CAPSL \ Call CAPSL to check whether CAPS LOCK is being pressed \ (if it is, the return value in A is the key number of \ CAPS LOCK, but with bit 7 set) PHP \ Retrieve the value of A we stored in Y, but making TYA \ sure the retrieval doesn't affect the flags PLP BPL P%+4 \ If the result of the call to CAPSL was positive, then \ CAPS LOCK isn't being pressed, so skip the next \ instruction ORA #%10000000 \ CAPS LOCK is being pressed, so set bit 7 of A TAX \ Copy A into X to return the key number of CAPS LOCK \ with bit 7 set
 RTS                    \ Return from the subroutine

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