Sense Insensibility

There's some serious hacking going on here. This code runs right at the start of BASIC, and it's job is to modify the IO code at 1100-1300 which reads input chars and prints output chars. The modifications make the code specific to whatever device the user has configured via the front panel. Actual modifications are :

1. Port numbers to use for read/write.
2. Bits to test for read/write readiness.
3. Conditional instructions to use after testing bits for read/write readiness.

ReadSense:	4D66	IN A,(FF)	Read sense switches.
	4D68	AND F0	Only interested in A15-A12.
	4D6A	RRCA
	4D6B	RRCA
	4D6C	CP 3C	No switches set, then return.
	4D6E	RET Z
	4D6F	CP 38	Only A12 set?
	4D71	SCF	Carry = 1.
	4D72	JP NZ,4D7F	If A15, A14, or A13 set then jump ahead.
	4D75	LD HL,5FFF	Spong! What on earth can be here??
	4D78	LD C,(HL)
	4D79	DEC HL
	4D7A	LD A,(HL)
	4D7B	AND F0
	4D7D	RRCA
	4D7E	RRCA
Use A15-A12 configuration in A as index into some table. The lowest possible index is 0 and the highest is 34, in multiples of 4.
	4D7F	PUSH AF	Preserve A15 - A12.
	4D80	LD L,A
	4D81	LD H,00
	4D83	LD DE,4DE4
	4D86	ADD HL,DE	HL=offset into IO table.
	4D87	LD A,(HL)	A=port #.
	4D88	INC HL
	4D89	LD D,(HL)	D=conditional jump opcode.
	4D8A	INC HL
	4D8B	LD B,(HL)	B=IO read test bit.
	4D8C	INC HL
	4D8D	LD E,(HL)	E=IO write test bit.
	4D8E	LD H,A	H=port #.
	4D8F	POP AF
	4D90	PUSH AF
	4D91	LD A,H	A=port #.
	4D92	JP C,4D96	Spong! Where is carry set?
	4D95	LD A,C
	4D96	LD (4DE2),A	Write port # to code below.
	4D99	POP AF	A=port #.
	4D9A	LD HL,4E00
	4D9D	PUSH HL	Push ??? address onto stack
	4D9E	LD C,FF
	4DA0	CP 10	If A15, A13, A12 all set
	4DA2	LD HL,0000
	4DA5	LD (116E),HL	Writes two NOPs into IO code. Why??
	4DA8	JP Z,4DBA
	4DAB	CP 08
	4DAD	RET NC	'Return' to 4E00.
	4DAE	ADD A,11
	4DB0	PUSH AF
	4DB1	LD A,03
	4DB3	CALL 4DE1
	4DB6	POP AF
	4DB7	JP 4DE1
	4DBA	XOR A
	4DBB	CALL 4DE1
	4DBE	CALL 4DDD
	4DC1	CALL 4DDD
	4DC4	CPL
	4DC5	LD C,01
	4DC7	CALL 4DDD
	4DCA	PUSH HL
	4DCB	LD HL,(4DE1)
	4DCE	LD L,DB
	4DD0	LD (116E),HL
	4DD3	POP HL
	4DD4	LD A,2C
	4DD6	DEC (HL)
	4DD7	CALL 4DE1
	4DDA	DEC (HL)
	4DDB	DEC (HL)
	4DDC	DEC (HL)
	4DDD	LD HL,4DE2
	4DE0	INC (HL)
	4DE1	OUT (10),A
	4DE3	RET

I/O configuration table

First byte = port #.
Second byte = conditional instruction: 0xCA = JP Z, 0xC2 = JP NZ.
Third byte = bit to test for IO read readiness.
Fourth byte = bit to test for IO write readiness.

4DE4	10 CA 01 02	A15, A14, A13, A12
4DE8	10 CA 01 02	A15, A14, A13
4DEC	00 C2 01 80	A15, A14, A12
4DF0	06 C2 01 80	A15, A14
4DF4	20 CA 80 80	A15, A13, A12
4DF8	04 CA 02 01	A15, A13
4DFC	24 CA 40 40	A15, A12

I/O Code Modification

Using values from the above table and code, this modifies the IO code in the 1100-1300 area to use selected ports, test bits, and conditional flow opcodes.

4E00	LD H,D	H=conditional jump opcode.
4E01	LD L,B	L=test bit.
4E02	LD (11A1),HL	IO_Read_ready code.
4E05	LD A,H
4E06	AND C8
4E08	LD H,A	H=0xC8 (RET Z) or 0xC0 (RET NZ).
4E09	LD (134F),HL	Write to elsewhere in IO code.
4E0C	XOR 0C
4E0E	LD H,A	H=0xC4 (CALL NZ) or 0xCC (CALL Z).
4E0F	LD (123E),HL	Write to elsewhere in IO code.
4E12	EX DE,HL
4E13	LD (1166),HL	IO_Write_ready code.
4E16	LD A,(4DE2)
4E19	LD (119F),A
4E1C	LD (134D),A
4E1F	LD (123C),A
4E22	INC A
4E23	LD (11A6),A
4E26	ADD A,C
4E27	LD (1164),A
4E2A	INC A
4E2B	LD (116C),A
4E2E	RET
4E2F	NOP
4E30	NOP