12 Jun
2000
12 Jun
'00
9:55 a.m.
Hi:
I'm not particularly good at C, so I'm trying to learn more as I add
tape image support to Claus Guiloi's Altair Emulator.
Anyway, I have a "C pointers" question regarding copying the tape
bits to the emulator memory. Here's some pseudo code...
//delcared in i8080.c MEMSIZE is virtual memory size in bytes
uchar Mem [MEMSIZE];
BOOL ReadTape (PSTR pstrFileName)
PSTR pstrBuffer ; //buffer for fread command
// make sure the file exists
//allocate memory to fit tape image length and load file
//determine file type (BIN or HEX)
//if BIN, make sure that the image fits into the emulator memory size
if (iLength > MEMSIZE){
OkMessage (hWnd, "Program too big to fit in memory!", szTitleName) ;
return FALSE;
}
// for binary, so get load address from user and copy to memory array
addr=MsgBox("Enter load address"); //obviously wrong. but it's only pseudo
code
// make sure that (load address + file size) <=65535
// copy binary image to the emulator memory
for (i = 0; i <= iLength; i++)
Mem[i+addr] = (uchar) *pstrBuffer+i;
return 0;
This is the important code. The Altair's memory is represented by an array
of type uchar and pstrBuffer is the file buffer used int he fread command.
My question is whether I'm doing the assignment right?
Thanks again for the help.
Rich
12 Jun
12 Jun
10:18 a.m.
New subject: Altair emulator code & C pointers
// copy binary image to the emulator memory
for (i = 0; i <= iLength; i++)
Mem[i+addr] = (uchar) *pstrBuffer+i;
return 0;
This is the important code. The Altair's memory is represented
by an array
of type uchar and pstrBuffer is the file buffer used int he fread command.
My question is whether I'm doing the assignment right?
You could potentially get weird behavior that way. Some compilers do
strange
things converting signed to unsigned values. Try this instead:
Mem[i+addr] = *((uchar *)(pstrBuffer+i));
or even:
Mem[i+addr] = ((uchar *)pstrBuffer)[i];
for clarity. Note, I tend to go overboard with parens, I've seen a number
of compilers and other programs which didn't do operator precedence
correctly.
11:10 a.m.
New subject: Altair emulator code & C pointers
On Mon, Jun 12, 2000 at 11:18:58AM -0400, Bill Sudbrink wrote:
things converting signed to unsigned values. Try this
instead:
Mem[i+addr] = *((uchar *)(pstrBuffer+i));
or even:
Mem[i+addr] = ((uchar *)pstrBuffer)[i];
This is not correct, the pstrBuffer element you are referring to is not a
pointer but an uchar.
The correct line should be:
Mem[addr+i] = (uchar) *(pstrBuffer+i);
Good luck,
Erik.
12:36 p.m.
New subject: Altair emulator code & C pointers
Mem[i+addr]
= *((uchar *)(pstrBuffer+i));
or even:
Mem[i+addr] = ((uchar *)pstrBuffer)[i];
This is not correct, the pstrBuffer element you are referring to is not a
pointer but an uchar. The correct line should be:
Mem[addr+i] = (uchar) *(pstrBuffer+i);
This will result in exactly the same potentially ambiguous behavior as his
original example. The compiler will probably deref the pointer into a
signed int with the twos complement sign bit extended into the higher bits.
Then the fun begins. If it takes the signed int and (logically) converts it
into an unsigned int and then tries to convert that to an unsigned char,
you potentially wind up with a significant overflow. The behavior at this
point is not well defined. You might get the desired answer, you might not.
Depends on who implemented the compiler, what the target architecture is,
etc.
11:46 a.m.
<PICKYPICKY>
for (i = 0; i <= iLength; i++)
Did you want the number of iterations to be iLength, or iLength +1 (what
you have there)?
Mem[i+addr] = (uchar) *pstrBuffer+i;
Did you want to add i to the pointer and retrieve what the resulting
pointer points to, or did you want to retrieve what the pointer points to,
and then add i to that value?
</PICKYPICKY>
--
Grumpy Ol' Fred cisin(a)xenosoft.com
12:32 p.m.
At 10:55 AM 6/12/00 -0400, you wrote:
[snip]
// copy binary image to the emulator memory
for (i = 0; i <= iLength; i++)
Mem[i+addr] = (uchar) *pstrBuffer+i;
return 0;
This is the important code. The Altair's memory is represented by an array
of type uchar and pstrBuffer is the file buffer used int he fread command.
My question is whether I'm doing the assignment right?
Hi Rich,
A possibly faster way to implement this loop is:
memcpy(Mem + addr, pstrBuffer, iLength);
return(0);
Regards,
Bob
2:58 p.m.
At 01:46 PM 6/12/00 -0400, you wrote:
I agree. I was going to suggest that but the original code
didn't show reading the data in so I didn't know if additional
checks were being made to insure it was valid before placing
it in "memory".
Regards,
Bob
A possibly
faster way to implement this loop is:
memcpy(Mem + addr, pstrBuffer, iLength);
Or, if it's binary, why not read directly into Mem and
skip the extra buffer all together. Something like:
fread (Mem + addr, 1, iLength, pFile);
2:34 p.m.
It was thus said that the Great Cini, Richard once stated:
Hi:
I'm not particularly good at C, so I'm trying to learn more as I add
tape image support to Claus Guiloi's Altair Emulator.
Anyway, I have a "C pointers" question regarding copying the tape
bits to the emulator memory. Here's some pseudo code...
//delcared in i8080.c MEMSIZE is virtual memory size in bytes
[ snipped --- NOTE: ``//'' is NOT a valid ANSI C comment delimeter, but
some C compilers will accept it]
I would be inclined to do the following:
#include <stdio.h>
#include <stddef.h>
#define MAXSIZE 65536L
#ifndef FALSE
# define FALSE 0
#endif
#ifndef TRUE
# define TRUE !FALSE
#endif
typedef size_t i8080addr;
int ReadTape (char *filename,void *dest,size_t maxsize);
i8080addr AskLoadAddress (void);
unsigned char Mem [MEMSIZE];
/* code ... */
loadaddr = AskLoadAddress();
if (loadaddr >= MAXSIZE) /* too large of an address */
return(FALSE);
rc = ReadTape(filename,Mem+loadaddr,MAXSIZE - loadaddr);
if (rc == FALSE)
return(rc);
/* rest of code */
Basically, read the file directly into the 8080 virtual memory space.
Saves moving it around afterwards.
But for your original question reguarding:
uchar Mem [MEMSIZE];
PSTR pstrBuffer ; //buffer for fread command
// copy binary image to the emulator memory
for (i = 0; i <= iLength; i++)
Mem[i+addr] = (uchar) *pstrBuffer+i;
This is the important code. The Altair's memory is represented by an array
of type uchar and pstrBuffer is the file buffer used int he fread command.
My question is whether I'm doing the assignment right?
The right hand side looks suspicious so I might make it read:
for (i = 0 ; i < iLength ; i++)
Mem[i + addr] = (uchar) (*(pstrBuffer + i));
Arrays are zero based, so an array that is 10 elements long uses indicies
from 0 to 9, so your original code would write one too
many bytes (and may
or may not crash the program, depending upon the system).
-spc (Have C compiler, will program)
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bill＠chipware.com -
bwit＠pobox.com -
cisin＠xenosoft.com -
ebrens＠dds.nl -
RCini＠congressfinancial.com -
spc＠armigeron.com