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028E: THE 'KEYBOARD SCANNING' SUBROUTINE
This very important subroutine is called by both the main keyboard subroutine (KEYBOARD) and the INKEY$ routine (S_INKEY).
In all instances the E register is returned with a value in the range of +00 to +27, the value being different for each of the forty keys of the keyboard, or the value +FF, for no-key.
The D register is returned with a value that indicates which single shift key is being pressed. If both shift keys are being pressed then the D and E registers are returned with the values for the CAPS SHIFT and SYMBOL SHIFT keys respectively.
If no key is being pressed then the DE register pair is returned holding +FFFF.
The zero flag is returned reset if more than two keys are being pressed, or neither key of a pair of keys is a shift key.
Input
Output
D Shift key pressed (+18 or +27), or +FF if no shift key pressed
E Other key pressed (+00 to +27), or +FF if no other key pressed
F Zero flag reset if an invalid combination of keys is pressed
KEY_SCAN 028E LD L,$2F The initial key value for each line will be +2F, +2E,..., +28. (Eight lines.)
0290 LD DE,$FFFF Initialise DE to 'no-key'.
0293 LD BC,$FEFE C=port address, B=counter.
Now enter a loop. Eight passes are made with each pass having a different initial key value and scanning a different line of five keys. (The first line is CAPS SHIFT, Z, X, C, V.)
KEY_LINE 0296 IN A,(C) Read from the port specified.
0298 CPL A pressed key in the line will set its respective bit, from bit 0 (outer key) to bit 4 (inner key).
0299 AND $1F
029B JR Z,KEY_DONE Jump forward if none of the five keys in the line are being pressed.
029D LD H,A The key-bits go to the H register whilst the initial key value is fetched.
029E LD A,L
KEY_3KEYS 029F INC D If three keys are being pressed on the keyboard then the D register will no longer hold +FF - so return if this happens.
02A0 RET NZ
KEY_BITS 02A1 SUB $08 Repeatedly subtract 8 from the present key value until a key-bit is found.
02A3 SRL H
02A5 JR NC,KEY_BITS
02A7 LD D,E Copy any earlier key value to the D register.
02A8 LD E,A Pass the new key value to the E register.
02A9 JR NZ,KEY_3KEYS If there is a second, or possibly a third, pressed key in this line then jump back.
KEY_DONE 02AB DEC L The line has been scanned so the initial key value is reduced for the next pass.
02AC RLC B The counter is shifted and the jump taken if there are still lines to be scanned.
02AE JR C,KEY_LINE
Four tests are now made.
02B0 LD A,D Accept any key value which still has the D register holding +FF, i.e. a single key pressed or 'no-key'.
02B1 INC A
02B2 RET Z
02B3 CP $28 Accept the key value for a pair of keys if the D key is CAPS SHIFT.
02B5 RET Z
02B6 CP $19 Accept the key value for a pair of keys if the D key is SYMBOL SHIFT.
02B8 RET Z
02B9 LD A,E It is however possible for the E key of a pair to be SYMBOL SHIFT - so this has to be considered.
02BA LD E,D
02BB LD D,A
02BC CP $18
02BE RET Return with the zero flag set if it was SYMBOL SHIFT and 'another key'; otherwise reset.
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