ALTAIR/altair.txt - emulators/simh - Rivoreo Source Code Repositories

 Altair 8800 Simulator
 =====================

 1. Background.

 	The MITS (Micro Instrumentation and Telemetry Systems) Altair 8800
 was announced on the January 1975 cover of Popular Electronics, which
 boasted you could buy and build this powerful computer kit for only $397.
 The kit consisted at that time of only the parts to build a case, power
 supply, card cage (18 slots), CPU card, and memory card with 256 *bytes* of
 memory.  Still, thousands were ordered within the first few months after the
 announcement, starting the personal computer revolution as we know it today.

 	Many laugh at the small size of the that first kit, noting there
 were no peripherals and the 256 byte memory size.  But the computer was an
 open system, and by 1977 MITS and many other small startups had added many
 expansion cards to make the Altair quite a respectable little computer. The
 "Altair Bus" that made this possible was soon called the S-100 Bus, later
 adopted as an industry standard, and eventually became the IEE-696 Bus.

 2. Hardware

 	We are simulating a fairly "loaded" Altair 8800 from about 1977,
 with the following configuration:

 	device		simulates
 	name(s)

 	CPU		Altair 8800 with Intel 8080 CPU board, 62KB
 			of RAM, 2K of EPROM with start boot ROM.
 	2SIO		MITS 88-2SIO Dual Serial Interface Board. Port 1
 			is assumed to be connected to a serial "glass
 			TTY" that is your terminal running the Simulator.
 	PTR		Paper Tape Reader attached to port 2 of the
 			2SIO board.
 	PTP		Paper Tape Punch attached to port 2 of the
 			2SIO board.  This also doubles as a printer
 			port.
 	DSK		MITS 88-DISK Floppy Disk controller with up
 			to eight drives.

 2.1 CPU

 	We have 2 CPU options that were not present on the original
 machine but are useful in the simulator.  We also allow you to select
 memory sizes, but be aware that some sample software requires the full
 64K (i.e. CP/M) and the MITS Disk Basic and Altair DOS require about
 a minimum of 24K.

 	SET CPU 8080	Simulates the 8080 CPU (normal)
 	SET CPU Z80	Simulates the later Z80 CPU [At the present time
 			this is not fully implemented and is not to be
 			trusted with real Z80 software]
 	SET CPU ITRAP	Causes the simulator to halt if an invalid 8080
 			Opcode is detected.
 	SET CPU NOITRAP	Does not stop on an invalid Opcode.  This is
 			how the real 8080 works.
 	SET CPU 4K
 	SET CPU 8K
 	SET CPU 12K
 	SET CPU 16K
 	  ......
 	SET CPU 64K	All these set various CPU memory configurations.
 			The 2K EPROM at the high end of memory is always
 			present and will always boot.

 The BOOT EPROM card starts at address 177400.  Jumping to this address
 will always boot drive 0 of the floppy controller.  If no valid bootable
 software is present there the machine crashes.  This is historically
 accurate behavior.

 The real 8080, on receiving a HLT (Halt) instruction, freezes the processor
 and only an interrupt or CPU hardware reset will restore it.  The simulator
 is alot nicer, it will halt but send you back to the simulator command line.

 CPU Registers include the following:

 	name	size	comments

 	PC	16	The Program Counter
 	A	8	The accumulator
 	BC	16	The BC register pair.  Register B is the high
 			8 bits, C is the lower 8 bits
 	DE	16	The DE register pair.  D is the top 8 bits, E is
 			the bottom.
 	HL	16	The HL register pair.  H is top, L is bottom.
 	C	1	Carry flag.
 	Z	1	Zero Flag.
 	AC	1	Auxillary Carry flag.
 	P	1	Parity flag.
 	S	1	Sign flag.
 	SR	16	The front panel switches.
 	BREAK	16	Breakpoint address (377777 to disable).
 	WRU	8	The interrupt character.  This starts as 005
 			(ctrl-E) but some Altair software uses this
 			keystroke so best to change this to something
 			exotic such as 035 (which is Ctl-]).


 2.2 The Serial I/O Card (2SIO)

 	This simple programmed I/O device provides 2 serial ports to the
 outside world, which could be hardware jumpered to support RS-232 plugs or a
 TTY current loop interface.  The standard I/O addresses assigned by MITS
 was 20-21 (octal) for the first port, and 22-23 (octal) for the second.
 We follow this standard in the Simulator.

 	The simulator directs I/O to/from the first port to the screen. The
 second port reads from an attachable "tape reader" file on input, and writes
 to an attachable "punch file" on output.  These files are considered a
 simple stream of 8-bit bytes.

 2.3 The 88-DISK controller.

 	The MITS 88-DISK is a simple programmed I/O interface to the MITS
 8-inch floppy drive, which was basically a Pertec FD-400 with a power
 supply and buffer board builtin.  The controller supports neither interrupts
 nor DMA, so floppy access required the sustained attention of the CPU.
 The standard I/O addresses were 10, 11, and 12 (octal), and we follow the
 standard.  Details on controlling this hardware are in the altair_dsk.c
 source file.


 3. Sample Software

 	Running an Altair in 1977 you would be running either MITS Disk
 Extended BASIC, or the brand new and sexy CP/M Operating System from Digital
 Research.  Or possibly, you ordered Altair DOS back when it was promised in
 1975, and are still waiting for it to be delivered in early 1977.

 	We have samples of all three for you to check out.  We can't go into
 the details of how they work, but we'll give you a few hints.


 3.1 CP/M Version 2.2

 	This version is my own port of the standard CP/M to the Altair.
 There were some "official" versions but I don't have them.  None were
 endorsed or sold by MITS to my knowledge, however.
 	To boot CP/M:

 	sim> attach dsk0 altcpm.dsk
 	sim> go 177400
 	62K CP/M VERSION 2.2 (ALTAIR 8800)
 	A>DIR

 	CP/M feels like DOS, sort of.  DIR will work.  I have included all
 the standard CP/M utilities, plus a few common public-domain ones.  I also
 include the sources to the customized BIOS and some other small programs.
 TYPE will print an ASCII file.  DUMP will dump a binary one.  LS is a better
 DIR than DIR.  ASM will assemble .ASM files to Hex, LOAD will "load" them to
 binary format (.COM).  ED is a simple editor, #A command will bring the
 source file to the buffer, T command will "type" lines, L will move lines,
 E exits the editor.  20L20T will move down 20 lines, and type 20.  Very
 DECish.  DDT is the debugger, SUBMIT is a batch-type command processor.
 A sample batch file that will assemble and write out the bootable CP/M
 image (on drive A) is "SYSGEN.SUB".  To run it, type "SUBMIT SYSGEN".


 3.2 MITS Disk Extended BASIC Version 4.1

 	This was the commonly used software for serious users of the Altair
 computer.  It is a powerful (but slow) BASIC with some extended commands to
 allow it to access and manage the disk.  There was no operating system it
 ran under.  To boot:

 	sim> attach dsk0 mbasic.dsk
 	sim> go 177400

 	MEMORY SIZE? [return]
 	LINEPRINTER? C [return]
 	HIGHEST DISK NUMBER? 0 [return]  (3 here = 4 drive system)
 	NUMBER OF FILES? 3 [return]
 	NUMBER OF RANDOM FILES? 2 [return]

 	44297 BYTES FREE
 	ALTAIR BASIC REV. 4.1
 	[DISK EXTENDED VERSION]
 	COPYRIGHT 1977 BY MITS INC.
 	OK
 	mount 0
 	OK
 	files


 3.3 Altair DOS Version 1.0

 	This was long promised but not delivered until it was almost
 irrelevant.  A short attempted tour will reveal it to be a dog, far inferior
 to CP/M.  To boot:

 	sim> attach dsk0 altdos.dsk
 	sim> go 177400

 	MEMORY SIZE? 64 [return]
 	INTERRUPTS? N [return]
 	HIGHEST DISK NUMBER? 0 [return]  (3 here = 4 drive system)
 	HOW MANY DISK FILES? 3 [return]
 	HOW MANY RANDOM FILES? 2 [return]

 	056769 BYTES AVAILABLE
 	DOS MONITOR VER 1.0
 	COPYRIGHT 1977 BY MITS INC
 	.mnt 0

 	.dir 0
	Altair 8800 Simulator
	=====================

	1. Background.

	The MITS (Micro Instrumentation and Telemetry Systems) Altair 8800
	was announced on the January 1975 cover of Popular Electronics, which
	boasted you could buy and build this powerful computer kit for only $397.
	The kit consisted at that time of only the parts to build a case, power
	supply, card cage (18 slots), CPU card, and memory card with 256 bytes of
	memory. Still, thousands were ordered within the first few months after the
	announcement, starting the personal computer revolution as we know it today.

	Many laugh at the small size of the that first kit, noting there
	were no peripherals and the 256 byte memory size. But the computer was an
	open system, and by 1977 MITS and many other small startups had added many
	expansion cards to make the Altair quite a respectable little computer. The
	"Altair Bus" that made this possible was soon called the S-100 Bus, later
	adopted as an industry standard, and eventually became the IEE-696 Bus.

	2. Hardware

	We are simulating a fairly "loaded" Altair 8800 from about 1977,
	with the following configuration:

	device simulates
	name(s)

	CPU Altair 8800 with Intel 8080 CPU board, 62KB
	of RAM, 2K of EPROM with start boot ROM.
	2SIO MITS 88-2SIO Dual Serial Interface Board. Port 1
	is assumed to be connected to a serial "glass
	TTY" that is your terminal running the Simulator.
	PTR Paper Tape Reader attached to port 2 of the
	2SIO board.
	PTP Paper Tape Punch attached to port 2 of the
	2SIO board. This also doubles as a printer
	port.
	DSK MITS 88-DISK Floppy Disk controller with up
	to eight drives.

	2.1 CPU

	We have 2 CPU options that were not present on the original
	machine but are useful in the simulator. We also allow you to select
	memory sizes, but be aware that some sample software requires the full
	64K (i.e. CP/M) and the MITS Disk Basic and Altair DOS require about
	a minimum of 24K.

	SET CPU 8080 Simulates the 8080 CPU (normal)
	SET CPU Z80 Simulates the later Z80 CPU [At the present time
	this is not fully implemented and is not to be
	trusted with real Z80 software]
	SET CPU ITRAP Causes the simulator to halt if an invalid 8080
	Opcode is detected.
	SET CPU NOITRAP Does not stop on an invalid Opcode. This is
	how the real 8080 works.
	SET CPU 4K
	SET CPU 8K
	SET CPU 12K
	SET CPU 16K
	......
	SET CPU 64K All these set various CPU memory configurations.
	The 2K EPROM at the high end of memory is always
	present and will always boot.

	The BOOT EPROM card starts at address 177400. Jumping to this address
	will always boot drive 0 of the floppy controller. If no valid bootable
	software is present there the machine crashes. This is historically
	accurate behavior.

	The real 8080, on receiving a HLT (Halt) instruction, freezes the processor
	and only an interrupt or CPU hardware reset will restore it. The simulator
	is alot nicer, it will halt but send you back to the simulator command line.

	CPU Registers include the following:

	name size comments

	PC 16 The Program Counter
	A 8 The accumulator
	BC 16 The BC register pair. Register B is the high
	8 bits, C is the lower 8 bits
	DE 16 The DE register pair. D is the top 8 bits, E is
	the bottom.
	HL 16 The HL register pair. H is top, L is bottom.
	C 1 Carry flag.
	Z 1 Zero Flag.
	AC 1 Auxillary Carry flag.
	P 1 Parity flag.
	S 1 Sign flag.
	SR 16 The front panel switches.
	BREAK 16 Breakpoint address (377777 to disable).
	WRU 8 The interrupt character. This starts as 005
	(ctrl-E) but some Altair software uses this
	keystroke so best to change this to something
	exotic such as 035 (which is Ctl-]).


	2.2 The Serial I/O Card (2SIO)

	This simple programmed I/O device provides 2 serial ports to the
	outside world, which could be hardware jumpered to support RS-232 plugs or a
	TTY current loop interface. The standard I/O addresses assigned by MITS
	was 20-21 (octal) for the first port, and 22-23 (octal) for the second.
	We follow this standard in the Simulator.

	The simulator directs I/O to/from the first port to the screen. The
	second port reads from an attachable "tape reader" file on input, and writes
	to an attachable "punch file" on output. These files are considered a
	simple stream of 8-bit bytes.

	2.3 The 88-DISK controller.

	The MITS 88-DISK is a simple programmed I/O interface to the MITS
	8-inch floppy drive, which was basically a Pertec FD-400 with a power
	supply and buffer board builtin. The controller supports neither interrupts
	nor DMA, so floppy access required the sustained attention of the CPU.
	The standard I/O addresses were 10, 11, and 12 (octal), and we follow the
	standard. Details on controlling this hardware are in the altair_dsk.c
	source file.


	3. Sample Software

	Running an Altair in 1977 you would be running either MITS Disk
	Extended BASIC, or the brand new and sexy CP/M Operating System from Digital
	Research. Or possibly, you ordered Altair DOS back when it was promised in
	1975, and are still waiting for it to be delivered in early 1977.

	We have samples of all three for you to check out. We can't go into
	the details of how they work, but we'll give you a few hints.


	3.1 CP/M Version 2.2

	This version is my own port of the standard CP/M to the Altair.
	There were some "official" versions but I don't have them. None were
	endorsed or sold by MITS to my knowledge, however.
	To boot CP/M:

	sim> attach dsk0 altcpm.dsk
	sim> go 177400
	62K CP/M VERSION 2.2 (ALTAIR 8800)
	A>DIR

	CP/M feels like DOS, sort of. DIR will work. I have included all
	the standard CP/M utilities, plus a few common public-domain ones. I also
	include the sources to the customized BIOS and some other small programs.
	TYPE will print an ASCII file. DUMP will dump a binary one. LS is a better
	DIR than DIR. ASM will assemble .ASM files to Hex, LOAD will "load" them to
	binary format (.COM). ED is a simple editor, #A command will bring the
	source file to the buffer, T command will "type" lines, L will move lines,
	E exits the editor. 20L20T will move down 20 lines, and type 20. Very
	DECish. DDT is the debugger, SUBMIT is a batch-type command processor.
	A sample batch file that will assemble and write out the bootable CP/M
	image (on drive A) is "SYSGEN.SUB". To run it, type "SUBMIT SYSGEN".


	3.2 MITS Disk Extended BASIC Version 4.1

	This was the commonly used software for serious users of the Altair
	computer. It is a powerful (but slow) BASIC with some extended commands to
	allow it to access and manage the disk. There was no operating system it
	ran under. To boot:

	sim> attach dsk0 mbasic.dsk
	sim> go 177400

	MEMORY SIZE? [return]
	LINEPRINTER? C [return]
	HIGHEST DISK NUMBER? 0 [return] (3 here = 4 drive system)
	NUMBER OF FILES? 3 [return]
	NUMBER OF RANDOM FILES? 2 [return]

	44297 BYTES FREE
	ALTAIR BASIC REV. 4.1
	[DISK EXTENDED VERSION]
	COPYRIGHT 1977 BY MITS INC.
	OK
	mount 0
	OK
	files


	3.3 Altair DOS Version 1.0

	This was long promised but not delivered until it was almost
	irrelevant. A short attempted tour will reveal it to be a dog, far inferior
	to CP/M. To boot:

	sim> attach dsk0 altdos.dsk
	sim> go 177400

	MEMORY SIZE? 64 [return]
	INTERRUPTS? N [return]
	HIGHEST DISK NUMBER? 0 [return] (3 here = 4 drive system)
	HOW MANY DISK FILES? 3 [return]
	HOW MANY RANDOM FILES? 2 [return]

	056769 BYTES AVAILABLE
	DOS MONITOR VER 1.0
	COPYRIGHT 1977 BY MITS INC
	.mnt 0

	.dir 0