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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 0 to 39, the value being different for each of the forty keys of the keyboard, or the value 255, 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 65535.
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.
D Shift key pressed (24 or 39), or 255 if no shift key pressed
E Other key pressed (0 to 39), or 255 if no other key pressed
F Zero flag reset if an invalid combination of keys is pressed
KEY_SCAN 00654 LD L,47 The initial key value for each line will be 47, 46,..., 40. (Eight lines.)
00656 LD DE,65535 Initialise DE to 'no-key'.
00659 LD BC,65278 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 00662 IN A,(C) Read from the port specified.
00664 CPL A pressed key in the line will set its respective bit, from bit 0 (outer key) to bit 4 (inner key).
00665 AND 31
00667 JR Z,KEY_DONE Jump forward if none of the five keys in the line are being pressed.
00669 LD H,A The key-bits go to the H register whilst the initial key value is fetched.
00670 LD A,L
KEY_3KEYS 00671 INC D If three keys are being pressed on the keyboard then the D register will no longer hold 255 - so return if this happens.
00672 RET NZ
KEY_BITS 00673 SUB 8 Repeatedly subtract 8 from the present key value until a key-bit is found.
00675 SRL H
00679 LD D,E Copy any earlier key value to the D register.
00680 LD E,A Pass the new key value to the E register.
00681 JR NZ,KEY_3KEYS If there is a second, or possibly a third, pressed key in this line then jump back.
KEY_DONE 00683 DEC L The line has been scanned so the initial key value is reduced for the next pass.
00684 RLC B The counter is shifted and the jump taken if there are still lines to be scanned.
Four tests are now made.
00688 LD A,D Accept any key value which still has the D register holding 255, i.e. a single key pressed or 'no-key'.
00689 INC A
00690 RET Z
00691 CP 40 Accept the key value for a pair of keys if the D key is CAPS SHIFT.
00693 RET Z
00694 CP 25 Accept the key value for a pair of keys if the D key is SYMBOL SHIFT.
00696 RET Z
00697 LD A,E It is however possible for the E key of a pair to be SYMBOL SHIFT - so this has to be considered.
00698 LD E,D
00699 LD D,A
00700 CP 24
00702 RET Return with the zero flag set if it was SYMBOL SHIFT and 'another key'; otherwise reset.
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