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[ The PC Guide | Systems and Components Reference Guide | Video Cards | Video Memory Function and Speed ]

Memory Bandwidth, Speed and Performance

It is important to realize that increasing the speed of the video memory does not, in general, mean that your video performance is going to improve dramatically. You are not going to see your frame rate in a favorite game double by replacing a DRAM video card with a VRAM one. See here for a whole section on performance issues.

Better memory technologies and faster memory access mean higher video bandwidth. Bandwidth is the total amount of memory that can be read at one time; it is a factor in overall video performance, as well as one of the factors that determine the ability of the card to support higher resolutions and color depths, at reasonable refresh rates. What this means is that a card using faster memory technologies will support running at 1280x1024 at 24 bits, while one that uses older DRAM will not.

The reference table below shows the necessary minimum memory bandwidth, in megabytes per second, required from the video memory in order to support a given combination of resolution, color depth and refresh rate. (Remember that support of a given refresh rate also requires support from a fast enough RAMDAC):

Resolution

Bit Depth

43.5 Hz (87 Interlaced)

60 Hz

72 Hz

80 Hz

85 Hz

90 Hz

100 Hz

320x200

8

2.7

3.6

4.4

4.9

5.1

5.5

6.1

16

5.3

7.3

8.8

9.7

10.4

11.0

12.2

24

8.0

11.0

13.2

14.7

15.5

16.5

18.3

640x480

8

12.8

17.5

21.1

23.5

24.9

26.3

29.3

16

25.5

35.2

42.2

46.9

49.8

52.7

58.6

24

38.2

52.7

63.3

70.3

74.7

79.1

87.9

800x600

8

19.9

27.5

33.0

36.6

38.9

41.2

45.8

16

39.9

54.9

65.9

73.2

77.8

82.4

91.6

24

59.7

82.4

98.9

109.9

116.7

123.6

137.3

1024x768

8

32.6

45.0

54.0

60.0

63.7

67.5

75.0

16

65.2

90.0

108.0

120.0

127.5

135.0

150.0

24

97.8

135.0

162.0

180.0

191.2

202.5

225.0

1280x1024

8

54.4

75.0

90.0

100.0

106.2

112.5

125.0

16

108.7

150.0

180.0

200.0

212.5

225.0

250.0

24

163.1

225.0

270.0

300.0

318.7

337.5

375.0

1600x1200

8

79.6

109.9

131.8

146.5

155.6

164.8

183.1

16

159.3

219.7

263.7

293.0

311.3

329.6

366.2

24

239.0

329.6

395.5

439.5

466.9

494.4

549.3

Note: The bandwidth figures above are calculated by multiplying the screen resolution dimensions to obtain the number of pixels per refresh, by the refresh rate to get pixels per second, and then by the number of bytes per pixel required for color information to get bytes per second. Then this number was divided by 1,048,576 (not 1,000,000) to obtain the value in MB/s.

Next: Memory Size and Memory Access


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