Re: Re: A86: Questions on Port 3, calc on, off, APD, etc
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Re: Re: A86: Questions on Port 3, calc on, off, APD, etc
It is not how the 16-bit segment and 16-bit offsets overlap that form the
1meg barrier. All 4 gigs of memory on a pc can be accessed with a 386 or
greater in protected mode by simply using a 32-bit pointer. The 1 meg
limit comes from the 8088/8086 having only 20 address lines for memory.
2^20 = 1 meg. The 286 had 24 address lines, and it can address 16 megs of
memory in pmode. 2^24 = 16 megs. And the 386 and later have 32 address
lines, hence the 4 gigs of ram. Not that many (if any) people have that
much ram on a pc, but 16 megs on a 286 is like 4 gigs on a PII, believe it
or not!
I'm glad someone agrees with my sentiment about programming on the pc vs.
calculator. The pc is very complicated and changes too fast. You can
write a cool game for the calc like sqrxz and impress everyone, or write
Wolf3d, Doom, or Quake for the PC and impress everyone. But what took
longer? (hmm...those three titles seem to come from the same company...)
But you pushed the platform to the limit in both cases, but on the PC it is
impossible to write everything in asm since it'd take too long and all the
fundamentals change when a new processor comes out.
Now, back to z80 on the calc--where the fun is!
At 08:17 PM 5/29/98 -0400, you wrote:
>
>>I don't know what book he was refering to, but I have read several on the
>>subject. Two that stand out are Peter Norton's Inside the PC, and The
>>Peter Norton PC Programming Bible. These books have much info on the Intel
>>based PC:
>>
>>Interrupt 2 on the PC, which has 256 vectored software interrupts, is the
>>NMI interrupt. This NMI bypasses the regular PIC interrupt controller. It
>>is used by the hardware to signal that memory has not been refreshed or is
>>otherwise corrupted. It is normally tied to a BIOS routine that gives a
>>"PARITY ERROR" message, then halts the computer. I've seen this on my old
>>8088 based IBM PC immediately after pulling the memory expansion card; I'd
>>forgotten to turn off the computer first. It also occurs when a device
>>conflicts with memory--like a game controller card.
>>
>>On IBM PS/2 systems, with Microchannel architecture, there is a Timer Mode
>>3 (0-2 are on all PC's) that is also tied to the NMI. It can be used by
>>the operating system as a watchdog to make sure a task like a timer tick
>>handler doesn't take too long and crash the entire system, since most ISR's
>>cannot be re-entered.
>>
>>But since this doesn't have much to do with A86 programming, I'll let you
>>read the books for yourselves.
>
>
>:-)
>
>Programming for the PC is kind of dull, because every program imaginable has
>already been made, and there is just too much competition. Plus ASM rarely
>applies to PC programming nowadays, except maybe for display drivers or
>whatever. It is nice programming for a platform that is challenging, but not
>overwhelming. It is nice squeezing the last drop of speed out of
>the Z80.. All the brute speed on the PC leads to bad programming habits and
>lack of optimization for many programs. Plus this Segment Offset crap just
>complicates programming. It is understandable to have memory divided on, for
>example, the TI-86 since the CPU can only address 64K at once, 2^8..... but
>damn come one a 32-bit processor? 2^32 = 4,294,967,296 bytes. If I recall a
>16-bit segment and a 16-bit offset overlap to form a 20-bit address which is
>where this 1 meg barrier comes from. I am not even sure how more then 1
>megabyte is accessed, anyone remember?
>
>Later,
> Matt
>
>
>
// David Phillips
// mailto:electrum@tfs.net
// AIM: electrum32
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