• Choose a category
• All
• Classic-Fiction

Figure 6-2 What do these numbers mean?

Organizing DRAM

Because of its low cost, high speed, and capability to contain a lot of data in a relatively small package, DRAM has been the standard RAM used in all computers—not just PCs—since the mid-1970s. DRAM can be found in just about everything, from automobiles to automatic bread makers.

The PC has very specific requirements for DRAM. The original 8088 processor had an 8-bit frontside bus. All the commands given to an 8088 processor were in discrete 8-bit chunks. You needed RAM that could store data in 8-bit (1-byte) chunks, so that each time the CPU asked for a line of code, the memory controller could put an 8-bit chunk on the data bus. This optimized the flow of data into (and out from) the CPU.

Although today’s DRAM chips may have widths greater than 1 bit, back in the old days all DRAM chips were 1 bit wide. That means you only had such sizes as 64 K × 1 or 256 K × 1—always 1 bit wide. So how was 1-bit-wide DRAM turned into 8-bit-wide memory? The answer was quite simple: Just take eight 1-bit-wide chips and electronically organize them with the memory controller chip to be eight wide. First, put eight 1-bit-wide chips in a row on the motherboard and then wire up this row of DRAM chips to the memory controller chip (which has to be designed to handle this) to make byte-wide memory (Figure 6-3). You just made eight 1-bit-wide DRAM chips look like a single 8-bit-wide DRAM chip to the CPU.

Figure 6-3 The MCC accessing data on RAM soldered onto the motherboard

Practical DRAM

Okay, before you learn more about DRAM, I need to make a critical point extremely clear. When you first saw the 8088’s machine language in Chapter 5, “Microprocessors,” all the examples in the “codebook” were exactly 1-byte commands. Figure 6-4 shows the codebook again—see how all the commands are 1 byte?

Figure 6-4 Codebook again

Well, the reality is slightly different. Most of the 8088 machine language commands are 1 byte, but a few more complex commands need 2 bytes. For example, the following command tells the CPU to move 163 bytes “up the RAM spreadsheet” and run whatever command is there. Cool, eh?

1110100110100011

The problem here is that the command is 2 bytes wide, not 1 byte. So how did the 8088 handle this? Simple—it just took the command 1 byte at a time. It took twice as long to handle the command because the MCC had to go to RAM twice, but it worked.

Okay, so if some of the commands are more than 1 byte wide, why didn’t Intel make the 8088 with a 16-bit frontside bus? Wouldn’t that have been better? Well, Intel did. Intel invented a CPU called the 8086. The 8086 actually predates the 8088 and was absolutely identical to the 8088 except for one small detail: it had a 16-bit frontside bus. IBM could have used the 8086 instead of the 8088 and used 2-byte-wide RAM instead of 1-byte-wide RAM. Of course, they would have needed to invent a memory controller chip that handled that kind of RAM (Figure 6-5).

Figure 6-5 Pumped-up 8086 MCC at work

Why didn’t Intel sell IBM the 8086 instead of the 8088? There were two reasons. First, nobody had invented an affordable MCC or RAM that handled 2 bytes at a time. Sure, chips were invented, but they were expensive and IBM didn’t think anyone would want to pay $12,000 for a personal computer. So IBM bought the Intel 8088, not the Intel 8086, and all our RAM came in bytes. But as you might imagine, it didn’t stay that way too long.

DRAM Sticks

As CPU data bus sizes increased, so too did the need for RAM wide enough to fill the bus. The Intel 80386 CPU, for example, had a 32-bit data bus and thus the need for 32-bit-wide DRAM. Imagine having to line up 32 one-bit-wide DRAM chips on a motherboard. Talk about a waste of space! Figure 6-6 shows motherboard RAM run amuck.

DRAM manufacturers responded by creating wider DRAM chips, such as, ×4, ×8, and ×16, and putting multiples of them on a small circuit board called a stick or module. Figure 6-7 shows an early stick, called a single