I don't understand what this is? It is not a hex or binary number

[David]

Hi guys, I need some help. 

What is this?  Why does it happen?

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;When I compile my code and debug it for errors, I just noticed something, what numbering is this?

;For example, if this is my code:

push 1 ;6A 01   It makes sense to me, however... what I am about to type below does not make sense
push 199; 68 99 01 00 00 ;I thought it would have been 68 C7, whats up, why is in that format?  Should it not be 68 C7? What exactly is this numbering format?


Thanks just curious what numbering format is being used,  It doesn't seem like hex or binary to me, if it were hex it would be 68 C7, and if it were binary it would be 68 11 00 01 11

If it's a shift, what is it being shifted by? 

PS: I am trying to make a compiler that will compile the following instructions:
ADD
CMP
SUB
MOV ;Having trouble with its hex values, ie Mov eax, 1000 has a weird value
NOP ;0x90

[dedndave]

the 2-byte form is only valid for byte-sized signed values (-128 to +127)
try this code and see what the assembler generates

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        push    -57
        pop     eax
then see what is in eax   :bg
eax should be FFFFFFC7h
it gets sign-extended to a dword

[David]

How would I convert it so that my assembler can compile it?  I had no problem making my assembler when all the functions were NOP lol, however I am having trouble with push, how would I make a function to generate the hex code of a push function that accepts one parameter?

What would be the formula for turning 199 into  99 01 00 00
and 100 into 00 10 00 00?

You said that it is in 2 byte form, how would I convert to that?

[clive]

What are you compiling/assembling/disassembling with?

If PUSH 199 is assembling as 68 99 01 00 00 it is interpreting 199 as hexidecimal, and encoding it as 4 bytes, low order first

68 Opcode
99 LSB
01
00
00 MSB


PUSH 012345678h ; 0x12345678

Would be 68 78 56 34 12

In pseudo code

X = 0xDEADBEEF;

B[0] = 0x68; Opcode
B[1] = (BYTE)((X >> 0) & 0xFF);
B[2] = (BYTE)((X >> 8) & 0xFF);
B[3] = (BYTE)((X >> 16) & 0xFF);
B[4] = (BYTE)((X >> 24) & 0xFF);

[dedndave]

it should be simple enough, David
if the operand evaluates to a value between -128 and +127, you can use the 2-byte form (6Ah opcode)
otherwise, you must push the full dword value (68h opcode)

[dedndave]

a word of caution, though..

in this example, the assembler should be able to use the 2-byte form (although some assemblers may not)

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SomeEquate EQU 12
.
.
.
        push    SomeEquate


in this example, the 5-byte form must always be used, as the value of "SomeEq" is unknown at assembly time

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SomeEq = 12
.
.
.
        push    SomeEq

[David]

thank you very much guys, you made it much more clear for me  :bg

[clive]

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Microsoft (R) Macro Assembler Version 6.15.8803     04/23/10 12:42:48
pushtest.asm      Page 1 - 1


        .386
        .MODEL FLAT,C
00000000         .CODE

00000000 start:
00000000  6A 01         push    1
00000002  6A 02         push    2
00000004  6A 7F         push    127
00000006  68 00000080         push    128
0000000B  68 000000C7         push    199
00000010  68 000000C8         push    200
00000015  68 000003E8         push    1000

0000001A  6A 7F         push    7Fh
0000001C  68 00000080         push    80h
00000021  68 00000199         push    199h

00000026  6A FF         push    -1
00000028  6A FE         push    -2
0000002A  68 FFFFFC18         push    -1000

0000002F  68 12345678         push    12345678h
00000034  68 DEADBEEF         push    0DEADBEEFh

        END start

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Disassembly

00000000                    _start:
00000000 6A01                   push    1
00000002 6A02                   push    2
00000004 6A7F                   push    7Fh
00000006 6880000000             push    80h
0000000B 68C7000000             push    0C7h
00000010 68C8000000             push    0C8h
00000015 68E8030000             push    3E8h
0000001A 6A7F                   push    7Fh
0000001C 6880000000             push    80h
00000021 6899010000             push    199h
00000026 6AFF                   push    0FFFFFFFFh
00000028 6AFE                   push    0FFFFFFFEh
0000002A 6818FCFFFF             push    0FFFFFC18h
0000002F 6878563412             push    12345678h
00000034 68EFBEADDE             push    0DEADBEEFh