ALTAIR/altair.txt

   1 Altair 8800 Simulator
   2 =====================
   3
   4 1. Background.
   5
   6         The MITS (Micro Instrumentation and Telemetry Systems) Altair 8800
   7 was announced on the January 1975 cover of Popular Electronics, which
   8 boasted you could buy and build this powerful computer kit for only $397.
   9 The kit consisted at that time of only the parts to build a case, power
  10 supply, card cage (18 slots), CPU card, and memory card with 256 *bytes* of
  11 memory.  Still, thousands were ordered within the first few months after the
  12 announcement, starting the personal computer revolution as we know it today.
  13
  14         Many laugh at the small size of the that first kit, noting there
  15 were no peripherals and the 256 byte memory size.  But the computer was an
  16 open system, and by 1977 MITS and many other small startups had added many
  17 expansion cards to make the Altair quite a respectable little computer. The
  18 "Altair Bus" that made this possible was soon called the S-100 Bus, later
  19 adopted as an industry standard, and eventually became the IEE-696 Bus.
  20
  21 2. Hardware
  22
  23         We are simulating a fairly "loaded" Altair 8800 from about 1977,
  24 with the following configuration:
  25
  26         device          simulates
  27         name(s)
  28
  29         CPU             Altair 8800 with Intel 8080 CPU board, 62KB
  30                         of RAM, 2K of EPROM with start boot ROM.
  31         2SIO            MITS 88-2SIO Dual Serial Interface Board. Port 1
  32                         is assumed to be connected to a serial "glass
  33                         TTY" that is your terminal running the Simulator.
  34         PTR             Paper Tape Reader attached to port 2 of the
  35                         2SIO board.
  36         PTP             Paper Tape Punch attached to port 2 of the
  37                         2SIO board.  This also doubles as a printer
  38                         port.
  39         DSK             MITS 88-DISK Floppy Disk controller with up
  40                         to eight drives.
  41
  42 2.1 CPU
  43
  44         We have 2 CPU options that were not present on the original
  45 machine but are useful in the simulator.  We also allow you to select
  46 memory sizes, but be aware that some sample software requires the full
  47 64K (i.e. CP/M) and the MITS Disk Basic and Altair DOS require about
  48 a minimum of 24K.
  49
  50         SET CPU 8080    Simulates the 8080 CPU (normal)
  51         SET CPU Z80     Simulates the later Z80 CPU [At the present time
  52                         this is not fully implemented and is not to be
  53                         trusted with real Z80 software]
  54         SET CPU ITRAP   Causes the simulator to halt if an invalid 8080
  55                         Opcode is detected.
  56         SET CPU NOITRAP Does not stop on an invalid Opcode.  This is
  57                         how the real 8080 works.
  58         SET CPU 4K
  59         SET CPU 8K
  60         SET CPU 12K
  61         SET CPU 16K
  62           ......
  63         SET CPU 64K     All these set various CPU memory configurations.
  64                         The 2K EPROM at the high end of memory is always
  65                         present and will always boot.
  66
  67 The BOOT EPROM card starts at address 177400.  Jumping to this address
  68 will always boot drive 0 of the floppy controller.  If no valid bootable
  69 software is present there the machine crashes.  This is historically
  70 accurate behavior.
  71
  72 The real 8080, on receiving a HLT (Halt) instruction, freezes the processor
  73 and only an interrupt or CPU hardware reset will restore it.  The simulator
  74 is alot nicer, it will halt but send you back to the simulator command line.
  75
  76 CPU Registers include the following:
  77
  78         name    size    comments
  79
  80         PC      16      The Program Counter
  81         A       8       The accumulator
  82         BC      16      The BC register pair.  Register B is the high
  83                         8 bits, C is the lower 8 bits
  84         DE      16      The DE register pair.  D is the top 8 bits, E is
  85                         the bottom.
  86         HL      16      The HL register pair.  H is top, L is bottom.
  87         C       1       Carry flag.
  88         Z       1       Zero Flag.
  89         AC      1       Auxillary Carry flag.
  90         P       1       Parity flag.
  91         S       1       Sign flag.
  92         SR      16      The front panel switches.
  93         BREAK   16      Breakpoint address (377777 to disable).
  94         WRU     8       The interrupt character.  This starts as 005
  95                         (ctrl-E) but some Altair software uses this
  96                         keystroke so best to change this to something
  97                         exotic such as 035 (which is Ctl-]).
  98
  99
 100 2.2 The Serial I/O Card (2SIO)
 101
 102         This simple programmed I/O device provides 2 serial ports to the
 103 outside world, which could be hardware jumpered to support RS-232 plugs or a
 104 TTY current loop interface.  The standard I/O addresses assigned by MITS
 105 was 20-21 (octal) for the first port, and 22-23 (octal) for the second.
 106 We follow this standard in the Simulator.
 107
 108         The simulator directs I/O to/from the first port to the screen. The
 109 second port reads from an attachable "tape reader" file on input, and writes
 110 to an attachable "punch file" on output.  These files are considered a
 111 simple stream of 8-bit bytes.
 112
 113 2.3 The 88-DISK controller.
 114
 115         The MITS 88-DISK is a simple programmed I/O interface to the MITS
 116 8-inch floppy drive, which was basically a Pertec FD-400 with a power
 117 supply and buffer board builtin.  The controller supports neither interrupts
 118 nor DMA, so floppy access required the sustained attention of the CPU.
 119 The standard I/O addresses were 10, 11, and 12 (octal), and we follow the
 120 standard.  Details on controlling this hardware are in the altair_dsk.c
 121 source file.
 122
 123
 124 3. Sample Software
 125
 126         Running an Altair in 1977 you would be running either MITS Disk
 127 Extended BASIC, or the brand new and sexy CP/M Operating System from Digital
 128 Research.  Or possibly, you ordered Altair DOS back when it was promised in
 129 1975, and are still waiting for it to be delivered in early 1977.
 130
 131         We have samples of all three for you to check out.  We can't go into
 132 the details of how they work, but we'll give you a few hints.
 133
 134
 135 3.1 CP/M Version 2.2
 136
 137         This version is my own port of the standard CP/M to the Altair.
 138 There were some "official" versions but I don't have them.  None were
 139 endorsed or sold by MITS to my knowledge, however.
 140         To boot CP/M:
 141
 142         sim> attach dsk0 altcpm.dsk
 143         sim> go 177400
 144         62K CP/M VERSION 2.2 (ALTAIR 8800)
 145         A>DIR
 146
 147         CP/M feels like DOS, sort of.  DIR will work.  I have included all
 148 the standard CP/M utilities, plus a few common public-domain ones.  I also
 149 include the sources to the customized BIOS and some other small programs.
 150 TYPE will print an ASCII file.  DUMP will dump a binary one.  LS is a better
 151 DIR than DIR.  ASM will assemble .ASM files to Hex, LOAD will "load" them to
 152 binary format (.COM).  ED is a simple editor, #A command will bring the
 153 source file to the buffer, T command will "type" lines, L will move lines,
 154 E exits the editor.  20L20T will move down 20 lines, and type 20.  Very
 155 DECish.  DDT is the debugger, SUBMIT is a batch-type command processor.
 156 A sample batch file that will assemble and write out the bootable CP/M
 157 image (on drive A) is "SYSGEN.SUB".  To run it, type "SUBMIT SYSGEN".
 158
 159
 160 3.2 MITS Disk Extended BASIC Version 4.1
 161
 162         This was the commonly used software for serious users of the Altair
 163 computer.  It is a powerful (but slow) BASIC with some extended commands to
 164 allow it to access and manage the disk.  There was no operating system it
 165 ran under.  To boot:
 166
 167         sim> attach dsk0 mbasic.dsk
 168         sim> go 177400
 169
 170         MEMORY SIZE? [return]
 171         LINEPRINTER? C [return]
 172         HIGHEST DISK NUMBER? 0 [return]  (3 here = 4 drive system)
 173         NUMBER OF FILES? 3 [return]
 174         NUMBER OF RANDOM FILES? 2 [return]
 175
 176         44297 BYTES FREE
 177         ALTAIR BASIC REV. 4.1
 178         [DISK EXTENDED VERSION]
 179         COPYRIGHT 1977 BY MITS INC.
 180         OK
 181         mount 0
 182         OK
 183         files
 184
 185
 186 3.3 Altair DOS Version 1.0
 187
 188         This was long promised but not delivered until it was almost
 189 irrelevant.  A short attempted tour will reveal it to be a dog, far inferior
 190 to CP/M.  To boot:
 191
 192         sim> attach dsk0 altdos.dsk
 193         sim> go 177400
 194
 195         MEMORY SIZE? 64 [return]
 196         INTERRUPTS? N [return]
 197         HIGHEST DISK NUMBER? 0 [return]  (3 here = 4 drive system)
 198         HOW MANY DISK FILES? 3 [return]
 199         HOW MANY RANDOM FILES? 2 [return]
 200
 201         056769 BYTES AVAILABLE
 202         DOS MONITOR VER 1.0
 203         COPYRIGHT 1977 BY MITS INC
 204         .mnt 0
 205
 206         .dir 0
 207
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