- Conceptual Overview
- Read-Only Memory (ROM)
- Random Access Memory (RAM)
- Cycles and Frequencies
- Summary—Basic Memory
- Cache Memory
- Memory Pages
- Rambus Memory (RDRAM)
- Double Data Rate SDRAM (DDR SDRAM)
- Video RAM (VRAM)
- Supplemental Information
- Packaging Modules
- Memory Diagnostics—Parity
- Exam Prep Questions
- Need to Know More?
This chapter is from the book
This chapter is from the book
This chapter is from the book
Supplemental Information
VRAM has been superseded by DDR SDRAM and Synchronous Graphics RAM (SGRAM). This is a specialized form of SDRAM that uses bit masking (writing to a specified bit plane without affecting the others) and block writes (filling a block of memory with a single color). Synchronous Graphics RAM uses very fast memory transfers. It also incorporates specific design changes for certain acceleration features built into video cards. SGRAM is still single-ported, unlike VRAM or WRAM, but offers performance similar to VRAM. SGRAM is typically used in moderate to high-end cards where performance is important, but very high resolution isn't required.
Multibank DRAM (MDRAM)
Multibank DRAM was invented by MoSys, specifically for use in graphics cards, and differs substantially in design from other types of video memory. Conventional memory designs use a single block of memory for the frame buffer. MDRAM breaks its memory up into multiple 32KB banks that can be accessed independently. This means that instead of the entire bandwidth being devoted to a single frame, smaller pieces can be processed in an overlapped system. This overlapping is called interleaving, and isn't the same as interlaced monitors, which we discuss in Chapter 9.
Given that other forms of video memory use these single blocks, video cards tend to be manufactured with increments of whole megabytes of memory, typically in 1MB, 2MB, 4MB, or 8MB, and so forth. A monitor running 1,024x768 resolution in true color (24 bits) uses 2.25MB of video memory for the frame buffer. That's more than 2MB, but the next step up is 4MB, leaving 1.75MB of wasted memory. MDRAM has no such restriction, allowing video cards to be manufactured with any amount of RAM, even exactly 2.25MB.
Earlier, we mentioned that VRAM and WRAM are dual-ported. Table 3.2 lists the various types of memory, along with the way they're ported. You probably won't need to know single or dual, but this may help put all the types of memory in one place.
Table 3.2 Types of Memory Used for Video Processing
|
Memory Type |
Ports |
|
Standard (FPM) DRAM |
Single |
|
EDO DRAM |
Single |
|
VRAM |
Dual |
|
WRAM |
Dual |
|
SGRAM |
Single |
|
MDRAM |
Single |