Need Guru's help...
ZXNet echo conference «code.zx»
Hello, valker
val> Imho; no way.;
No, somehow it’s possible. You need to check the byte back like this, for example, 10 is the previous one
check the byte for a 1-byte command, before it - for a 2-byte one, even before it -
to 3-byte. The last one that matches will be the previous command.
Hello, valker
maximum instruction length in z80 4 bytes
from this you can count -
test -4 = 4 byte command?
if not then test -3 = 3 byte command?
etc.
Hello, Strunov
Str> There is a disassembler program. How to organize scrolling up.
Str> More precisely, how to determine the number of bytes in the command coming from above??
Imho; no way.;
Hello, valker
val> Imho; no way.;
Let me clarify, in the general case - no way. We step back 1 byte - there is a command - but not
the fact that this is exactly it (possibly part of the data), we step back 2 bytes - there
command with 1 byte parameter (or 2 byte command), step by 3 bytes
back - there is a command with a 2-byte parameter or a 3-byte command.
Automatically decide which of the 3 is correct without analyzing the progress
program is impossible. Analysis may be difficult (for example, transitions along
calculated addresses, when the data for calculations becomes known only at
stage of the program).
Hello, valker
The simplest way is to step back a certain number of bytes (~4) and
try to disassemble (that is, determine the length of the command). If between
the end of this disassembled command and the beginning of the current line is not
gap, then we consider that we have received the address. Otherwise, we try to disassemble
one byte further.
But this method is inaccurate (IMHO it is used in STS).
The most accurate way is to disassemble several instructions back. Also
we step back a certain number of bytes (~16) and try to disassemble several
commands until we reach the original one (without overlaps or gaps). Otherwise, we walk
forward one byte. This is used in the US.
If the program is unmodified (it’s in ROM or something) and the disasm is on a powerful
machine is done, then you can simply make a bitmap of commands and just run
on her.
Hello, All
There is a disassembler program. How to organize scrolling up. More precisely
how to determine the number of bytes in the command coming from above??
Hello Jerry
jer> maximum command length in z80 4 bytes
Nope...
Don't forget about duplicating/alternating index register prefixes
Hello Vitamin
dd fd dd fd are you talking about these? they are stacked and only the last one is taken into account
and if dd cb or dd ed then there are still commands in the total of 4 bytes
I wrote a tracer, I had no commands longer than 4 bytes
well, or name a command like this :)
Hello, Strunov
checking back only 4 bytes is not enough. Example...
ld de,#FD00
ldhl,#FFFF
(x)
if you check back from point (x) - 4 bytes back it will be LD IY,#FFFF
IMHO, the design with a backspace of 16 bytes is just right.
Hello, Strunov
in general it is impossible. for example, the data in front of the cursor is 21 21 21 3E FF
can be dissembled as
┌─- code ───
some_byte db 21
my_procedure ld hl,#3E21
rst #38
cursor;
└──; code ───
or how
┌─- code ───
my_procedure ld hl,#2121
ld a,#FF
cursor;
└──; code ───
depends on whether control is ever transferred to the first byte in
chain or second
Hello SMT
Speaking of birds. Is there at least one disasm that correctly decodes
calculator commands?
Hello Jerry
jer> dd fd dd fd are you talking about these? they are stacked and only the last one is taken into account
jer>
jer> and if dd cb or dd ed then there are still commands in the total of 4 bytes
jer> I wrote a tracer; I had no commands longer than 4 bytes
jer>
jer> or name this command
This is exactly what it’s about. They are joined, the last one is taken into account, but still the length of the command
consists of the sum of ALL prefixes. And only in such cases, in all others
just 4 bytes..
Hello, Jukov
Juk> For what purpose are you interested, if it’s not a secret?
I want to write a disassembler...
Hello, Strunov
Str> There is a disassembler program. How to organize scrolling up.
Str> More precisely, how to determine the number of bytes in the command coming from above??
For what purpose are you interested, if it’s not a secret?