Routines
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0F34: scan_keyboard: Loop through keyboard rows to detect a keypress and return return key code.
Used by the routine at get_keypress.
scan_keyboard 0F34 LD D,$08 B to b11111110 and C to $FE.
0F36 LD BC,$FEFE
scan_keyboard_0 0F39 IN A,(C) Read one keyboard row.
0F3B DEC D Decrement counter.
0F3C OR $E0 If any key bit is low jump to scan_keyboard_1.
0F3E CP $FF
0F40 JR NZ,scan_keyboard_1
0F42 RLC B Else rotate 0 bit left in B.
0F44 LD A,D If loop counter is > 0 loop back to scan_keyboard_0.
0F45 AND A
0F46 JR NZ,scan_keyboard_0
0F48 XOR A Else return 0 (no key pressed).
0F49 RET
scan_keyboard_1 0F4A LD A,B Copy row mask into A.
0F4B LD BC,$0800 Set B to 8 as a loop counter and C to 0 as a row count.
scan_keyboard_2 0F4E SRL A Shift A right.
0F50 JR NC,scan_keyboard_3 If the 0 bit carried out jump to scan_keyboard_3.
0F52 INC C Else increment the row count in C.
0F53 DJNZ scan_keyboard_2 Decrement loop counter and loop back to scan_keyboard_2 if greater than zero.
0F55 XOR A Else return 0 (no key pressed).
0F56 RET
scan_keyboard_3 0F57 LD A,C Multiply row count by 5 and push onto stack.
0F58 ADD A,A
0F59 ADD A,A
0F5A ADD A,C
0F5B PUSH AF
0F5C LD BC,$00FE Read from all keyboard rows.
0F5F IN A,(C)
0F61 LD B,$05 Set B to 5 as a loop counter, and C to 0 as a column count.
0F63 LD C,$00
scan_keyboard_4 0F65 SRL A Shift input byte left.
0F67 JR NC,scan_keyboard_5 If 0 carried out jump to scan_keyboard_5.
0F69 INC C Else increment the column count in C.
0F6A DJNZ scan_keyboard_4 Decrement loop counter and loop back to scan_keyboard_4 if > 0.
0F6C POP AF Else balance stack and return 0 (no key pressed).
0F6D XOR A
0F6E RET
scan_keyboard_5 0F6F POP AF Retrieve row count from stack and add column count.
0F70 ADD A,C
0F71 LD HL,keyboard_lookup_table Add result to keyboard_lookup_table as offset into keyboard look up table.
0F74 LD B,$00
0F76 LD C,A
0F77 ADD HL,BC
0F78 LD A,(HL) Return the key code.
0F79 RET
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