Atari 520 ST +
The Atari 520ST+ is the same machine as the Atari 520ST but it has 1MByte of RAM. The second RAM bacnk is soldered directly on top of the first one, with the RAS/CAS pins bent up and connected to the MMU Memory controller.
Atari ST - Brief History
At the Winter Las Vegas Consumer Electronics Show of 1985, the Atari 130ST was introduced. ST stands for Sixteen bit data-bus and Thirty-two address-bus. Atari, after it was bought by Jack Tramiel, originally had contracted the Amiga Corporation to manufacture a 16-bit computer, but the contract was dissolved due to legal issues. Atari then rushed to get a machine out to compete with the release of the Comodore Amiga.
The Atari ST line came with many new features over previous Atari computers:
- 16 bit processor, 68000 CPU
- Midi Interface
- Graphical User Interface OS
- High graphics resulution in color
The new ST systems came with several dedicated coprocessors that enabled enhanced sound, graphics and memory management:
- MFP 68901 - Interrupt handler
- Yamaha YM 2149 - Programable Sound Generator (AY-3-8910 compatible)
- Shifter - Custom video processor
- GLUE - Custom Memory Manager (MMU)
The Atari130 was a demonstration only prototype, and never released. Due to it's small memory size, the OS did not function properly and the machine was quickly replaced with the Atari 260ST. This computer came with 512KByte of memory, but due to the large need of the OS (192KByte) and the memory need of the enhanced graphics, only 64KByte of free RAM was left.
The new operating system was called TOS, the Tramiel Operating System. The OS was basically a port of CP/M for the 68000 processor. Also interesting is that the high level TOS calls, called GemDOS, were all compatible with the DOS calls for the PC (DOS INT21h calls).
The GUI was called GEM, which stands for Graphical Environment Mamnager. The OS was provided on disk for the early ST models, which limited the free RAM left, and later it was baked into 6 32KByte ROM chips when the OS was fully finished.