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James Gregory
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11-05.md
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11-05.md
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@ -10,8 +10,8 @@ anything but ideal for a 286. For one thing, most display adapters are
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8-bit devices, although newer adapters are 16-bit in nature. One
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consequence of that is that only 1 byte can be read or written per
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access to display memory; word-sized accesses to 8-bit devices are
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automatically split into 2 separate byte-sized accesses by the AT’s bus.
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Another consequence is that accesses are simply slower; the AT’s bus
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automatically split into 2 separate byte-sized accesses by the AT's bus.
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Another consequence is that accesses are simply slower; the AT's bus
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inserts additional wait states on accesses to 8-bit devices since it
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must assume that such devices were designed for PCs and may not run
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reliably at AT speeds.
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@ -25,9 +25,9 @@ it this way: If **REP MOVSW** on a PC can use more than half of all
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available accesses to display memory, then how much faster can code
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running on a 286 or 386 possibly run when accessing display memory?
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That’s right—less than twice as fast.
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That's right—less than twice as fast.
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In other words, instructions that access display memory won’t run a
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In other words, instructions that access display memory won't run a
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whole lot faster on ATs and faster computers than they do on PCs. That
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explains one of the two viewpoints expressed at the beginning of this
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section: The display adapter cycle-eater is just about the same on
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@ -40,7 +40,7 @@ performance of instructions that access display memory to the *maximum*
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performance of those instructions. Instructions that access display
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memory receive many more wait states when running on a 286 than they do
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on an 8088. Why? While the 286 is capable of accessing memory much more
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often than the 8088, we’ve seen that the frequency of access to display
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often than the 8088, we've seen that the frequency of access to display
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memory is determined not by processor speed but by the display adapter
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itself. As a result, both processors are actually allowed just about the
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same maximum number of accesses to display memory in any given time. By
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@ -50,19 +50,19 @@ the 8088.
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And that explains the second viewpoint expressed above regarding the
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display adapter cycle-eater vis-a-vis the 286 and 386. The display
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adapter cycle-eater, as measured in cycles lost to wait states, is
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indeed much worse on AT-class computers than it is on the PC, and it’s
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indeed much worse on AT-class computers than it is on the PC, and it's
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worse still on more powerful computers.
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------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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 *How bad is the display adapter cycle-eater on an AT? It’s this bad: Based on my (not inconsiderable) experience in timing display adapter access, I’ve found that the display adapter cycle-eater can slow an AT—or even a 386 computer—to near-PC speeds when display memory is accessed.*
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 *How bad is the display adapter cycle-eater on an AT? It's this bad: Based on my (not inconsiderable) experience in timing display adapter access, I've found that the display adapter cycle-eater can slow an AT—or even a 386 computer—to near-PC speeds when display memory is accessed.*
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------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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I know that’s hard to believe, but the display adapter cycle-eater gives
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I know that's hard to believe, but the display adapter cycle-eater gives
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out just so many display memory accesses in a given time, and no more,
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no matter how fast the processor is. In fact, the faster the processor,
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the more the display adapter cycle-eater hurts the performance of
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instructions that access display memory. The display adapter cycle-eater
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is not only still present in 286/386 computers, it’s worse than ever.
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is not only still present in 286/386 computers, it's worse than ever.
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What can we do about this new, more virulent form of the display adapter
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cycle-eater? The workaround is the same as it was on the PC: Access
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@ -73,15 +73,15 @@ display memory as little as you possibly can.
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The 286 and 386 offer a number of new instructions. The 286 has a
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relatively small number of instructions that the 8088 lacks, while the
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386 has those instructions and quite a few more, along with new
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addressing modes and data sizes. We’ll discuss the 286 and the 386
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addressing modes and data sizes. We'll discuss the 286 and the 386
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separately in this regard.
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The 286 has a number of instructions designed for protected-mode
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operations. As I’ve said, we’re not going to discuss protected mode in
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operations. As I've said, we're not going to discuss protected mode in
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this book; in any case, protected-mode instructions are generally used
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only by operating systems. (I should mention that the 286’s protected
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only by operating systems. (I should mention that the 286's protected
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mode brings with it the ability to address 16 MB of memory, a
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considerable improvement over the 8088’s 1 MB. In real mode, however,
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considerable improvement over the 8088's 1 MB. In real mode, however,
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programs are still limited to 1 MB of addressable memory on the 286. In
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either mode, each segment is still limited to 64K.)
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@ -104,7 +104,7 @@ constant number of bits.
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#### New Instructions and Features: The 386 {#Heading14}
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The 386 is somewhat more complex than the 286 regarding new features.
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Once again, we won’t discuss protected mode, which on the 386 comes with
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Once again, we won't discuss protected mode, which on the 386 comes with
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the ability to address up to 4 gigabytes per segment and 64 terabytes in
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all. In real mode (and in virtual-86 mode, which allows the 386 to
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multitask MS-DOS applications, and which is identical to real mode so
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@ -112,10 +112,10 @@ far as MS-DOS programs are concerned), programs running on the 386 are
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still limited to 1 MB of addressable memory and 64K per segment.
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The 386 has many new instructions, as well as new registers, addressing
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modes and data sizes that have trickled down from protected mode. Let’s
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modes and data sizes that have trickled down from protected mode. Let's
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take a quick look at these new real-mode features.
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Even in real mode, it’s possible to access many of the 386’s new and
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Even in real mode, it's possible to access many of the 386's new and
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extended registers. Most of these registers are simply 32-bit extensions
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of the 16-bit registers of the 8088. For example, EAX is a 32-bit
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register containing AX as its lower 16 bits, EBX is a 32-bit register
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@ -125,15 +125,15 @@ segment registers: FS and GS.
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The 386 also comes with a slew of new real-mode instructions beyond
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those supported by the 8088 and 286. These instructions can scan data on
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a bit-by-bit basis, set the Carry flag to the value of a specified bit,
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sign-extend or zero-extend data as it’s moved, set a register or memory
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sign-extend or zero-extend data as it's moved, set a register or memory
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variable to 1 or 0 on the basis of any of the conditions that can be
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tested with conditional jumps, and more. (Again, beware: Many of these
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complex 386-specific instructions are slower than equivalent sequences
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of simple instructions on the 486 and especially on the Pentium.) What’s
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of simple instructions on the 486 and especially on the Pentium.) What's
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more, both old and new instructions support 32-bit operations on the
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386. For example, it’s relatively simple to copy data in chunks of 4
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bytes on a 386, even in real mode, by using the **MOVSD** (“move string
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double”) instruction, or to negate a 32-bit value with **NEG eax**.
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386. For example, it's relatively simple to copy data in chunks of 4
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bytes on a 386, even in real mode, by using the **MOVSD** ("move string
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double") instruction, or to negate a 32-bit value with **NEG eax**.
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------------------------ --------------------------------- --------------------
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[Previous](11-04.html) [Table of Contents](index.html) [Next](11-06.html)
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