Version analysis of RDKEY

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

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).

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

Master
   Y                    Y is preserved


 Other entry points:

   RDKEY-1              Only scan the keyboard for valid BCD key numbers

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

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 FILLKL             \ Call FILLKL 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 TRTB%,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

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