2. Initialization

Once BASIC has been loaded from disk, execution begins right at the bottom of memory.

2.1 Jump forward

A simple jump to the initialisation code.

	0000	DI
	0001	JMP IO_Init(4E37)

2.2 I/O Init

todo.

EasterEgg:	4E31	LD HL,5012	Print "WRITTEN BY MICRO-SOFT..."
	4E34	CALL 21A8	Print "WRITTEN BY MICRO-SOFT..."
IO_Init:	4E37	LD HL,521D
	4E3A	LD SP,HL
	4E3B	LD (06E5),HL
	4E3E	LD (0AD3),HL
	4E41	CALL 4D66
	4E44	LD HL,FFFF
	4E47	LD (06E7),HL
	4E4A	XOR A
	4E4B	LD (06E4),A
	4E4E	CALL 17BD
	4E51	LD (0A8B),A
	4E54	LD (06D7),A
	4E57	INC HL
	4E58	LD (06D9),HL
	4E5B	LD (06DB),HL
	4E5E	LD (06DD),HL
	4E61	LD HL,0AA0
	4E64	LD (0A9E),HL

2.3 "MEMORY SIZE? "

todo.

	4E67	LD HL,503B	Print "MEMORY SIZE"
	4E6A	CALL 21A8
	4E6D	CALL 0E36
	4E70	JP C,4E67
	4E73	RST 10H
	4E74	CP 41	If user enters 'A', jump back to show the Easter Egg.
	4E76	JP Z,EasterEgg(4E31)	If user enters 'A', jump back to show the Easter Egg.
	4E79	OR A	If user provided an input, jump ahead.
	4E7A	JP NZ,Mem_given(4E90)
User has given no input, so we have to find the top of usable RAM by writing as high as we can.
	4E7D	LD HL,51D7	51D7 is lowest address of non-BASIC RAM.
TestMemByte:	4E80	INC HL
	4E81	LD A,H	If we've reached address 0000 then we've successfully tested the whole of addressable RAM (64K) so can jump to GotMem.
	4E82	OR L
	4E83	JP Z,GotMem(4EA1)
	4E86	LD A,(HL)	Read a byte.
	4E87	CPL	Invert it.
	4E88	LD (HL),A	Write it back.
	4E89	CP (HL)	Did it work?
	4E8A	JP Z,TestMemByte(4E80)	If so, loop back.
	4E8D	JP GotMem(4EA1)
User has specified number of Kilobytes(?check?). Not sure what's happening here.
Mem_given:	4E90	LD HL,0989	poss. HL = input addr.
	4E93	CALL 1C64	poss. Convert input to integer (in DE).
	4E96	LD A,(HL)
	4E97	OR A
	4E98	JP NZ,0BC3	poss. Jump to "SYNTAX ERROR".
	4E9B	LD A,D
	4E9C	OR A
	4E9D	JP Z,14DF
	4EA0	EX DE,HL
At this point HL holds the address of the last byte of usable RAM, +1.
GotMem:	4EA1	DEC HL	HL = address of last byte of usable RAM.
	4EA2	PUSH HL
	4EA3	LD DE,FF9D	DE = -99.
	4EA6	POP HL
	4EA7	DEC HL	HL = address of last word of usable RAM.
	4EA8	LD (0A9C),HL	Store last_word somewhere.
	4EAB	ADD HL,DE	HL = HL - 99.
	4EAC	JP NC,0B9D	Spong! Inconceivable jump surely?...
	4EAF	DEC HL	HL = last_word - 100.
	4EB0	PUSH HL	Keep it on the stack.
	4EB1	LD A,84	No idea what this bit is for...
	4EB3	LD (06DF),A
	4EB6	LD A,70
	4EB8	LD (06E0),A

2.4 "LINEPRINTER? "

Asks user to identify the printer. Only acceptable answers are 'Q', 'C', or 'O'. If user does not give one of these, the question is repeated.

	4EBB	LD HL,4F07	Print "LINEPRINTER"
	4EBE	CALL 21A8
	4EC1	CALL 0E36
	4EC4	JP C,4EBB
	4EC7	RST 10H
	4EC8	CP 51	A='Q'?
	4ECA	JP Z,PrinterQ(4F13)
	4ECD	CP 43	A='C'?
	4ECF	JP Z,PrinterC(4EED)
	4ED2	CP 4F	A='O'?
	4ED4	JP NZ,4EBB	Jump back cos no valid answer given.
PrinterO:	4ED7	LD DE,512A	DE = address of PrinterDriver_O.
	4EDA	LD C,AE	C = size of PrinterDriver_O.
	4EDC	LD A,01
	4EDE	OUT (02),A
	4EE0	LD A,50
	4EE2	LD (06DF),A
	4EE5	LD A,38
	4EE7	LD (06E0),A
	4EEA	JP CopyDriver(4EF9)
PrinterC:	4EED	LD DE,50A2	DE = address of PrinterDriver_C.
	4EF0	LD C,89	C = size of PrinterDriver_C.
	4EF2	LD A,11
	4EF4	OUT (03),A
	4EF6	XOR A
	4EF7	OUT (02),A
Here we copy the 'driver' for the given printer into place, much as modern OS'es load printer drivers today. The source for both printer drivers is right up in memory and it is safe to assume it will be reused for applications.
CopyDriver:	4EF9	LD HL,3C2D	HL = address of PrinterDriver
	4EFC	LD A,(DE)
	4EFD	LD (HL),A
	4EFE	INC HL
	4EFF	INC DE
	4F00	DEC C
	4F01	JP NZ,4EFC
	4F04	JP HighestDisk(4F38)
	4F07	"LINEPRINTER\0"
PrinterQ:	4F13	XOR A
	4F14	OUT (30),A
	4F16	OUT (32),A
	4F18	OUT (34),A
	4F1A	OUT (36),A
	4F1C	OUT (35),A
	4F1E	LD A,FF
	4F20	OUT (31),A
	4F22	OUT (33),A
	4F24	OUT (37),A
	4F26	LD A,04
	4F28	OUT (30),A
	4F2A	OUT (32),A
	4F2C	OUT (34),A
	4F2E	OUT (36),A
	4F30	LD A,FC
	4F32	OUT (37),A
	4F34	LD A,FA
	4F36	OUT (37),A

2.5 "HIGHEST DISK NUMBER? "

todo.

HighestDisk:	4F38	LD HL,4FBB	Print "HIGHEST DISK NUMBER" and get input.
	4F3B	CALL 4FF5	Print "HIGHEST DISK NUMBER" and get input.
	4F3E	LD (0786),A
	4F41	LD HL,4E2F	HL=0x4E2F + 0x27 * (highest_disk_number + 1).
	4F44	LD DE,0027	HL=0x4E2F + 0x27 * (highest_disk_number + 1).
	4F47	INC A
	4F48	ADD HL,DE
	4F49	DEC A
	4F4A	JP NZ,4F48

2.6 "HOW MANY FILES? "

todo.

	4F4D	PUSH HL	Preserve highest_disk.
	4F4E	LD HL,4FCF	Print "HOW MANY FILES" and get input.
	4F51	CALL 4FF5
	4F54	POP HL	HL = highest_disk.
	4F55	LD (0827),HL	Store it somewhere.
	4F58	LD DE,0829
	4F5B	LD (0849),A	Store file_count somewhere
I think we're initialising a file table here: A table of file_count+1 entries, each entry
	4F5E	INC A
	4F5F	LD BC,008A	BC=0x8A, poss. file descriptor size?
	4F62	EX DE,HL
	4F63	LD (HL),E
	4F64	INC HL
	4F65	LD (HL),D
	4F66	INC HL
	4F67	EX DE,HL
	4F68	ADD HL,BC
	4F69	DEC A
	4F6A	JP NZ,4F62
	4F6D	LD (0813),HL

2.7 "HOW MANY RANDOM FILES? "

todo.

4F70	LD HL,4FDE	Print "HOW MANY RANDOM FILES" and get input.
4F73	CALL 4FF5	Print "HOW MANY RANDOM FILES" and get input.
4F76	LD (0812),A
4F79	OR A
4F7A	INC A
4F7B	CALL 30C0
4F7E	INC HL
4F7F	LD (06E9),HL
4F82	EX (SP),HL	HL=HIMEM
4F83	LD DE,521D	If HIMEM < 0x521D then jump to "OUT OF MEMORY" error.
4F86	RST 20H	If HIMEM < 0x521D then jump to "OUT OF MEMORY" error.
4F87	JP C,0B9D
4F8A	POP DE	DE = rand_file
4F8B	LD SP,HL	SP = HIMEM.
4F8C	LD (06E5),HL
4F8F	LD (0AD3),HL
4F92	EX DE,HL
4F93	CALL 0B8E	This func. seems to check that there is enough stack space.
4F96	LD A,E	HL = DE - HL
4F97	SUB L
4F98	LD L,A
4F99	LD A,D
4F9A	SBC H
4F9B	LD H,A
4F9C	LD BC,FFF0	HL = HL - 0x10.
4F9F	ADD HL,BC
4FA0	CALL 17BD
4FA3	CALL 4661
4FA6	LD HL,5047
4FA9	CALL 21A8
4FAC	LD HL,21A8
4FAF	LD (0C63),HL
4FB2	LD HL,0BA2
4FB5	LD (0002),HL
4FB8	JP 4D4C
4FBB	"HIGHEST DISK NUMBER\0"
4FCF	"HOW MANY FILES\0"
4FDE	"HOW MANY RANDOM FILES\0"
4FF4	POP HL
4FF5	PUSH HL
4FF6	CALL 21A8
4FF9	CALL 0E36
4FFC	JP C,4FF4
4FFF	RST 10H
5000	CALL 1C64
5003	DEC HL
5004	RST 10H
5005	JP NZ,4FF4
5008	LD HL,000F
500B	RST 20H
500C	JP C,4FF4
500F	POP AF
5010	LD A,E
5011	RET
5012	"\r\n\nWRITTEN FOR ROYALTIES BY MICRO-SOFT\r\n\0"
503B	"MEMORY SIZE\0"
5047	" BYTES FREE\r\nALTAIR BASIC REV. 4.1\r\n[DISK EXTENDED VERSION]\r\nCOPYRIGHT 1977 BY MITS INC.\r\n\0"
50A2 - 5129		Source for PrinterDriver_C
512A - 51??		Source for PrinterDriver_O