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Merge pull request #32 from thp/milliseconds

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Fix time unit: Milliseconds instead of microseconds
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James Gregory 2019-08-13 21:21:09 +10:00 • committed by GitHub
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@ -878,8 +878,8 @@ the PC must be completely refreshed about once every four milliseconds
in order to ensure the integrity of the data it stores. Obviously, it's in order to ensure the integrity of the data it stores. Obviously, it's
highly desirable that the memory in the PC retain the correct data highly desirable that the memory in the PC retain the correct data
indefinitely, so each DRAM chip in the PC *must* always be refreshed indefinitely, so each DRAM chip in the PC *must* always be refreshed
within 4 µs of the last refresh. Since there's no guarantee that a given within 4 ms of the last refresh. Since there's no guarantee that a given
program will access each and every DRAM block once every 4 µs, the PC program will access each and every DRAM block once every 4 ms, the PC
contains special circuitry and programming for providing DRAM refresh. contains special circuitry and programming for providing DRAM refresh.
#### How DRAM Refresh Works in the PC #### How DRAM Refresh Works in the PC
@ -900,8 +900,8 @@ purpose of refreshing the DRAM; the data that is read isn't used.)
The 256 addresses accessed by the refresh DMA accesses are arranged so The 256 addresses accessed by the refresh DMA accesses are arranged so
that taken together they properly refresh all the memory in the PC. By that taken together they properly refresh all the memory in the PC. By
accessing one of the 256 addresses every 15.08 µs, all of the PC's DRAM accessing one of the 256 addresses every 15.08 µs, all of the PC's DRAM
is refreshed in 256 x 15.08 µs, or 3.86 µs, which is just about the is refreshed in 256 x 15.08 µs, or 3.86 ms, which is just about the
desired 4 µs time I mentioned earlier. (Only the first 640K of memory is desired 4 ms time I mentioned earlier. (Only the first 640K of memory is
refreshed in the PC; video adapters and other adapters above 640K refreshed in the PC; video adapters and other adapters above 640K
containing memory that requires refreshing must provide their own DRAM containing memory that requires refreshing must provide their own DRAM
refresh in pre-AT systems.) refresh in pre-AT systems.)
@ -1223,7 +1223,7 @@ display, and even with the display adapter cycle-eater it just doesn't
take that long to manipulate 4,000 bytes. Even if the display adapter take that long to manipulate 4,000 bytes. Even if the display adapter
cycle-eater were to cause the 8088 to take as much as 5µs per display cycle-eater were to cause the 8088 to take as much as 5µs per display
memory access—more than five times normal—it would still take only memory access—more than five times normal—it would still take only
4,000x 2x 5µs, or 40 µs, to read and write every byte of display memory. 4,000x 2x 5µs, or 40 ms, to read and write every byte of display memory.
That's a lot of time as measured in 8088 cycles, but it's less than the That's a lot of time as measured in 8088 cycles, but it's less than the
blink of an eye in human time, and video performance only matters in blink of an eye in human time, and video performance only matters in
human time. After all, the whole point of drawing graphics is to convey human time. After all, the whole point of drawing graphics is to convey
@ -1261,7 +1261,7 @@ seriously impact code performance, even as measured in human time.
For example, if we assume the same 5 µs per display memory access for For example, if we assume the same 5 µs per display memory access for
the EGA's high-resolution graphics mode that we assumed for text mode, the EGA's high-resolution graphics mode that we assumed for text mode,
it would take 26,000 x 2 x 5 µs, or 260 µs, to scroll the screen once in it would take 26,000 x 2 x 5 µs, or 260 ms, to scroll the screen once in
the EGA's high-resolution graphics mode, mode 10H. That's more than the EGA's high-resolution graphics mode, mode 10H. That's more than
one-quarter of a second—noticeable by human standards, an eternity by one-quarter of a second—noticeable by human standards, an eternity by
computer standards. computer standards.