PC card
The PC card is a series of cards designed by Aleph One and released by Acorn Computers Ltd. It is a development of the older Aleph One 386PC/486PC Expansion Card, providing PC compatibility for RiscPC systems.
Description
All RiscPC PC cards connect to the second processor slot on the motherboard. The basic design consists of a 486-class CPU, a Gemini I or Gemini II ASIC, and a level 2 cache. Unlike the older podules, the PC card can access memory on the RiscPC motherboard directly rather than requiring onboard RAM.
Variants
PC cards were sold in a number of configurations. The following are known to exist :
- ACA42 - TI 486SXL/40 (running at 33 MHz), Gemini I, 128kB cache
- ACA53 - IBM 486DX2/66, Gemini II, 128kB cache
- ACA56 - TI 486DX4/100, Gemini II, 128kB cache
- ACA57 - IBM 5x86C/100, Gemini II, 128kB cache
- CJE59 - AMD 5x86/133, Gemini II, 512kB cache
Technical details
Gemini maps up 32 MB of RAM into the 486 address space, consisting of up to 8 physically contiguous sections. This provides the 486 with direct access of RiscPC motherboard RAM. A level 2 cache is provided to improve performance; this is write-through on Gemini I and write-back on Gemini II.
Gemini also internally implements the PC DMA, PIC and PIT controllers; these were implemented by the SCAMP chip on the older podules. Accesses to memory or IO areas outside of these peripherals or the mapped memory will halt the PC CPU and the access will be visible to the ARM through status registers implemented in Gemini. The !PC support software polls the status registers and emulates accesses to the relevant areas.
Gemini supports emulation of DMA operations. The ARM side reads and writes data to Gemini registers and the DMA controller transfers data to and from PC memory appropriately. This is most commonly used for sound emulation, which was not possible on the older podules.
Video and disk IO are emulated by the ARM host and are slower than on a real PC. To alleviate this, Aleph One provided video accelerated drivers for Windows 3.x and 95.
A performance issue exists due to IOMD's inability to support 16-bit accesses. 16-bit writes from the 486 have to be broken up into pairs of 8-bit writes; this noticeably affects performance of 16-bit software. Gemini II alleviates this somewhat by implementing a write-back level 2 cache, which reduces the number of writes going to main memory.
Images
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ACA57 (front)
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ACA57 (back)
Documents
- RiscPC 486 Card User Guide (1994) at Chris's Acorns
- RiscPC x86 Card User Guide (1995) at Chris's Acorns