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This is the recovered Machine Room from 2005. Please don't expect wonders. The look and feel of this site is nine years old, and so is its code. Some of the functionality has been recovered while the rest of the site is modernised and restructured. Watch this space!
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IBMPC

[Image: ]
A 1981 IBM PC shows off its Selectric-like keyboard and its monochrome monitor, blissfully unaware of its future as a home computer.
223x240 JPEG file (8.48 KiB). Contributed by alexios (more pictures like this). Unrated (0 votes).
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No known owners (I still own one | I used to own one). I have seen one.
Search Google or Ebay United States for IBM PC (more links, auctions & searches)
CPU
Intel's 8088 running at 4.77 MHz. 16 bits bits. Rumour has it IBM was considering using the Motorola 68000, but Intel had a host of ready support chips for the 8088 CPU (as it was signal-compatible to the 8080). Motorola didn't (unfortunately to some).

The 8088 is a 16-bit CPU with an 8-bit data bus and 20-bit address bus (i.e. it can address a total of 1 Mbyte of memory). It was preferred over the full 16-bit 8086 because it allowed for simpler external design of the machine.

ROM
40 KiB including the system's Basic Input/Output System (BIOS) and Microsoft's BASIC interpreter. A maximum of 128 kbytes of ROM could be added by add-on cards.
RAM
Initially, IBM wanted the machine to have 16 kbytes of RAM, like the {:IBM 5100'), but Microsoft convinced the design team to go for a minimum of 64 KiB. The motherboard has enough RAM chip sockets to allow memory configurations of 128 KiB, 192 KiB and 256 KiB. IBM expansion cards offered further expansion to 384 KiB, 512 KiB and 640 KiB. Lotus, Intel and Microsoft later agreed on a standard called LIM EMS (Lotus/Intel/Microsoft Expanded Memory Specification) that allowed memory expansion without limit. However, memory was seen by the system in 16 kbyte windows, which seriously hindered speed. EMS memory expansion cards are long forgotten now, but EMS (with all its bletcherity) is still emulated by DOS-based IBM PC compatibles.
Appearance
This is an IBM case with all the connotations. heavy, big desktop case. Made of black-painted steel and with a white and grey plastic and metal top, this case can survive anything. In fact, many have. The case has two black plastic blanking plates where floppy drives may be installed as an option (though most of the machines depicted in IBM ads actually had two floppy drives). There is an IBM-like Big Red Power Switch at the far end of the right side of the unit (so you really have to try to turn it on).

The case sports various openings to allow air circulation. At the rear, there are six metal blanking plates that cover the machine's six expansion slots. There are also three circular openings for DIN-type plugs. One is for the keyboard; the other for the tape recorder (yes, Pentium owners: tape recorder). The third is normally covered and is there for future expansion. Typically, when installing a third, external floppy drive, this would serve as a mount for the floppy's power plug. The left third of the rear panel of the IBM PC is occupied by the power supply (a noisy critter) with its big fan.

Keyboard
Based on IBM's Selectric typewriter. This is a detachable keyboard, connected to the machine via a black spiral cable. The keyboard is metal and very heavy but, like the PC's case, will withstand any treatment (you pay a lot for IBM products, but at least you know that you can bury your office equipment under a salt-water lake and, six centuries later, they'll still boot).

The keyboard has 84 clicky keys (but I simply love their feeling and feedback). Here are all the keys you'd expect to find on a PC keyboard. The ten function keys are in a 2 column formation at the left of the keyboard. There is a numeric keypad, but no separate cursor or editing keys. Instead, there is a NumLock key to change the numeric keypad into an array of editing keys. There is a Control key at the right place (on the home row), a Caps Lock key at the right place (to the right of the space bar, away from careless fingers), an Escape key at the right place (at the top left corner of the alphanumeric keyboard) and the list goes on and on. Of course, later keyboards weren't anything as standard as this one. IBM defines new standards; they don't obey existing ones. Also, there are a couple of other special keys like PrSc (Print Screen), Scroll Lock (temporarily stops output to the screen — not under MS-DOS, though) and Pause (the same as Scroll Lock I guess). There are no LEDs to show the state of the three Lock keys. The Enter key is too far away from the typist's fingers and is too small (normal-sized key-cap, like all the keys on this keyboard).

Display
The IBM PC comes with one of two display options. There is no display circuitry on the main board, so the user has to add either of the two display cards:
  1. The Monochrome Display Adapter (MDA). This one provides a single monochrome (2 colours) text display mode, at 80x25 with characters rendered in a 9x14 matrix. A green monochrome monitor is supplied. The card displays characters in one of five attributes: normal, bold (brighter text), inverse video, underline and blinking. Most of the attributes cannot be combined. The Motorola 6845 CRT controller is used to generate the image.
  2. The Colour Graphics Adapter (CGA) comes with a colour monitor. It is capable of the following display modes:
    • Text mode at 80x25x16. The colours are the standard eight video colours with eight dimmer versions. The dim version of yellow is brown/orange; the dark version of white is light grey. Black is the dim version of dark grey. The CGA can only do one special attribute, blinking text (inverse and bright may be emulated by using the colours; underline is impossible without direct manipulation of the graphics chip, the Motorola 6845 CRT controller. Characters rendered in an 8x8 matrix.
    • Text resolution at 40x25x16. Same as the previous mode otherwise.
    • Graphics Resolution at 320x200x4, chosen out of two predefined palettes. The first and most common is black, white, cyan and magenta; the second is black, yellow, green and red. Text is rendered in the usual 8x8 matrix, for a text resolution of 40x25x4.
    • High resolution graphics at 640x200x2 (black and white only). Text resolution at 80x25x2.
    • There is also an additional mode unsupported by the BIOS. It offers a graphics resolution of 160x100x16.

    The CGA connected to its monitor through a 9-pin D plug. There was also an RCA output for a composite monitor (unlike the MDA). Internally, there was a connector for a light-pen.

Both display cards come with 32 kbytes of framebuffer RAM. The unused memory (when doing text modes, for example) is reclaimed by using a ’paged’ technique, where the card keeps eight different frames in its memory and can display any of them at will.

The character set is fixed for both cards, since it comes in ROM. It includes a complete ASCII set plus 160 other special characters, including block graphics, grey shades, frame characters, international accented characters and general purpose obscure semi-graphics characters. These seem to have originated from the character set of some Wang word-processing system.

Third party video adapters were soon created for the 5150 to cover the huge holes left by IBM's ones. One of the classics was the Hercules Graphics Adapter. This was exactly like the MDA, but also supported a graphics mode of 720x350. Unfortunately, this was not supported in the BIOS. Later third party cards were dual and emulated both CGA and MDA using the same monitor. This allowed the user to run games on the CGA part of the card and serious applications on the Hercules/MDA part (since it had better monochrome graphics and more legible characters).

All of these cards came with a bonus parallel port (the 6845 had built-in hardware for this).

Audio
Simple beeper, driven by a timer so that the CPU didn't have to occupy itself with producing sounds.
Peripheral Memory
None by default (the tape port was used instead). An IBM optional controller card allowed the use of one to three full height, Single Sided Double Density 5.25-inch, 160 kbyte floppy drives, for a total of 160 KiB, 320 KiB or 480 KiB.

The third drive was connected externally. Later floppy drives had a capacity of 180 kbytes (SSDD), 320 kbytes (DSDD) or 360 kbytes (DSDD). Later yet, a 720 kbyte Double-Sided Double-Density 3.5-inch floppy drive could be added, for yet more combinations.

Given these floppy drives, the maximum storage capacity for the IBM 5150 was 1.41 MiB, using two 360 kbyte floppy drives and one 720 kbyte external 3.5 inch drive, but that was much later than the PC's release date.

I/O
Minimal facilities on the mainboard plus the pre-installed display card: Using add-on cards, almost anything could be added, including a real time clock, serial ports, parallel ports, Ethernet, modems, etc.