Video RAM Cacheable – The Tech ARP BIOS Guide

Video RAM Cacheable - The Tech ARP BIOS Guide

Video RAM Cacheable

Common Options : Enabled, Disabled

 

Quick Review of Video RAM Cacheable

The Video RAM Cacheable feature aims to boost VGA graphics performance by using the processor’s Level 2 cache to cache the 64 KB VGA graphics memory area from A0000h to AFFFFh.

If this BIOS feature is enabled, the VGA graphics memory area will be cached by the processor’s Level 2 cache. This speeds up accesses to the VGA graphics memory area.

If this BIOS feature is disabled, the VGA graphics memory area will not be cached by the processor’s Level 2 cache.

From what we have discussed so far, it sounds like caching the VGA graphics memory area is logically the way to go. Caching the VGA graphics memory area will definitely speed up VGA graphics performance by caching accesses to the graphics memory area.

However, reality is far less ideal. For one thing, VGA modes are hardly used at all these days. For compatibility reason, VGA is still used in Windows XP’s Safe Mode. It is also used in real mode DOS, if you still use that. Other than that, there is no more use for VGA modes. If VGA graphics modes are not used, no benefit can possibly be realized by enabling this BIOS feature.

Even if you use DOS modes a lot, is there even a point in caching the VGA graphics memory area for better performance? Even the slowest computer today is more than capable of handling VGA graphics with ease. In short, caching the VGA graphics memory area will not bring any noticeable advantage.

On the other hand, caching this memory area will cost you some processor performance. Because some of the processor’s Level 2 cache is being diverted to cache the VGA graphics memory area, there is less to keep the processor supplied with data. Consequently, the processor’s performance suffers.

Therefore, it is highly recommended that you disable the Video RAM Cacheable feature. There is no reason to enable it even if you use real mode DOS a lot or work a lot in Windows Safe Mode.

 

Details of Video RAM Cacheable

The Upper Memory Area (UMA) is a 384KB block of memory at the top of the first megabyte of memory that is reserved for the system’s use in DOS. A portion of this Upper Memory Area is reserved as video RAM memory.

The video RAM memory area is a 128KB block from A0000h to BFFFFh. Of this 128KB, the first half (A0000h-AFFFFh) is reserved for use in VGA graphics mode. The other half is used for monochrome text mode (B0000h-B7FFFh) and colour text mode (B8000h-BFFFFh). This video RAM memory area is the only portion of the graphics card’s memory that the processor has direct access to in VGA mode.

The graphics card and the processor use this memory area to write pixel data when the computer is operating in VGA mode. This is why all VGA graphics modes take up less than 64KB of memory. The most common VGA mode is mode 0x13 which has a resolution of 320 x 200 in 256 colours. This mode uses up exactly 64,000 bytes of memory and fits nicely into the 64KB block from A0000h to AFFFFh.

The Video RAM Cacheable feature aims to boost VGA graphics performance by using the processor’s Level 2 cache to cache the 64 KB VGA graphics memory area from A0000h to AFFFFh.

If this BIOS feature is enabled, the VGA graphics memory area will be cached by the processor’s Level 2 cache. This speeds up accesses to the VGA graphics memory area.

If this BIOS feature is disabled, the VGA graphics memory area will not be cached by the processor’s Level 2 cache.

From what we have discussed so far, it sounds like caching the VGA graphics memory area is logically the way to go. Caching the VGA graphics memory area will definitely speed up VGA graphics performance by caching accesses to the graphics memory area. This is great for those old DOS games although it won’t do anything for VGA text modes.

However, reality is far less ideal. For one thing, VGA modes are hardly used at all these days. For compatibility reason, VGA is still used in Windows XP’s Safe Mode. It is also used in real mode DOS, if you still use that. Other than that, there is no more use for VGA modes. If VGA graphics modes are not used, no benefit can possibly be realized by enabling this BIOS feature.

Even if you use DOS modes a lot, is there even a point in caching the VGA graphics memory area for better performance? Even the slowest computer today is more than capable of handling VGA graphics with ease. In short, caching the VGA graphics memory area will not bring any noticeable advantage.

On the other hand, caching this memory area will cost you some processor performance. Because some of the processor’s Level 2 cache is being diverted to cache the VGA graphics memory area, there is less to keep the processor supplied with data. Consequently, the processor’s performance suffers.

If the use of the processor’s Level 2 cache can bring about significant improvement in the performance of the graphics subsystem, it would have been worth it. Unfortunately, the VGA graphics modes are rarely used at all.

Even when used, there is little or no real benefit in caching the memory area. The Video RAM Cacheable BIOS feature just wastes the processor’s Level 2 cache on something that cannot possibly improve the system’s graphics performance.

Therefore, it is highly recommended that you disable the Video RAM Cacheable feature. There is no reason to enable it even if you use real mode DOS a lot or work a lot in Windows Safe Mode.

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