ALTAIR 8800

Robert Relyea – Alex Tavares

Background● Microcomputer released in 1974

● Commissioned by Popular Electronics and designed by Ed Roberts, president of MITS Incorporated

● Released as a complete kit for $439 or assembled for $621 (for base system)

● Significantly more powerful than available hobbyist computers

● Use of 8080 CPU rather than 8008

● Significantly cheaper than other machines using 8080 CPU

● The 8080 alone cost $300

System Architecture● Utilizes a motherboard with 100-pin slot connectors in parallel

• Up to 18 cards fit in the case, 4 slots available by default

● Pin functions include:

• Unregulated power

• 16-bit bidirectional address lines

• Two 8-bit unidirectional data lines

• Eight vector interrupt lines

• Status / Control / Clock lines

● Known as the “Altair Bus” or “S-100 Bus” by third parties

• Became the standard for home computers until early 1980s

• Modified to become IEEE-696 in 1982

System Architecture● A typical system includes:

• CPU card● Front panel

• Memory card(s)

• Peripheral card(s)● I/O● Storage

● Use of third party cards was common

Intel 8080 – Overview● Processor the Altair 8800 was designed for

● 8-bit processor with support for some 16-bit operations

● 2MHz max clock speed

● Implemented with 6µm NMOS

• Required 12V, 5V, and -5V rails due to poor electrical design

● 64KiB max addressable memory

Intel 8080 – Architecture● Seven 8-bit data registers

• One dedicated accumulator

• Six storage registers

● Can be utilized as three 16-bit registers

● 8-bit status register

● Sign, Zero, Parity, Carry, Aux Carry

● 16-bit stack pointer

● 16-bit program counter

A Flags

B C

D E

H L

Stack Pointer

Program Counter

Intel 8080 – Architecture● 8-bit instructions

● Capable of 16-bit operations

• Increment / decrement register pairs

• Add register pairs to registers HL

● Immediate operands

• Represented by up to two bytes after the instruction

<― 8 bits ―>

Opcode

Low Address

High Address

Intel 8080 – I/O● Supports dedicated input and output operations

• 256 addressable ports● Utilize address and data lines, but adds a pin to

designate memory or I/O operation

• Altair implementation used primarily memory-mapping

● Exception is 8 “sense switches” and LEDs on front panel

● Designed to use “Support Chips”

• External modules for PIT, Interrupt controllers, etc.

Zilog Z80● Microprocessor with greater capabilities than the Intel 8080

• Usually run at 4MHz (limitation of Altair Bus)

• Required only 5V rail

• Common aftermarket upgrade

● Binary compatible with Intel 8080, syntactically different ISA

• Ex: 'MOV A,D' (8080) synonymous with 'LD A, D' (Z80)

● “Alternate Registers” increase number of available data registers

• Must toggle between alternate registers

● Two dedicated 16-bit index registers

Front Panel

● Connected to CPU card

● Capable of executing instructions directly on the CPU

● AUX switches are disconnected

● CLEAR switch is used only by add-on cards

Memory● Upper bits of address were hard-wired per board via jumpers

● On-board stall circuitry (forces PRDY low)

● On-board protection flip-flop

● MITS offered 1K SRAM and 4K DRAM on launch

• Base system offered a 1K board with 256 bytes populated

● DRAM “cycle steals” to refresh, causing potential data loss

• Third party SRAM became commonly used

Peripherals + I/O● Peripherals are added in the form of additional cards

● Allowed for improved functionality such as:

• Serial and parallel I/O (used for TTY / paper tape systems)

• Audio cassette interfacing

• Disk control (floppy disks, hard disks)

• Real-time clocks

• Relay control

• Math co-processing

• Speech synthesis

• Modems

• Video generation + output

Peripherals + I/O● Peripherals are memory-mapped with hard-wired address bits

(similar to memory)

● Many external devices required start-up code (“bootstrap loader”) to utilize

• PROM cards could be installed to automate start-up tasks (“Turnkey” system)

● In everyday use, external devices were often used for writing and loading programs (BASIC, CP/M)

Legacy● High demand but poor availability caused first

'clone' machines to appear (IMSAI 8080)

● Often cited as the turning point of the personal computer revolution

● First computer to sell in large quantities

● Set standards for future microcomputers

● Popularity led to widespread software development (BASIC, CP/M, IMDOS)

● Led to the founding of 'Micro-Soft'

● Bill Gates and Paul Allen developed Altair BASIC upon hearing about the machine

Sources‒ 8080/8085 Assembly Language Programming Manual

‒ Altair 8800 Operators Manual

‒ Altair 8800 Theory of Operation Manual

‒ pc-history.org/altair.htm

‒ retrotechnology.com/herbs_stuff/altair_first.htm

‒ s100computers.com

‒ z80.eu/z80itself.html

‒ z80.info/z80brief.htm