[SOLVED] Referencing the low byte of a constant.

stf92 · 07-10-2011, 11:29 AM

Nasm 0.98.39

Hi:
Suppose I have this constant in an assembly language source program:

FREQ equ 1532

And I want to load the AL register with the low byte of FREQ. Made by hand, as 1532 = 0x05fc, it would be

mov al,0xfc (AT&T mov $0xfc,al)

In MASM (ms-dos) this was done

MOV AL,LOW(FREQ)

Does anybody know of a way of doing this in nasm?

Sergei Steshenko · 07-10-2011, 12:36 PM

Quote:

Originally Posted by stf92

Nasm 0.98.39

Hi:
Suppose I have this constant in an assembly language source program:

FREQ equ 1532

And I want to load the AL register with the low byte of FREQ. Made by hand, as 1532 = 0x05fc, it would be

mov al,0xfc (AT&T mov $0xfc,al)

In MASM (ms-dos) this was done

MOV AL,LOW(FREQ)

Does anybody know of a way of doing this in nasm?

How about using bitwise AND with 0xff ?

stf92 · 07-10-2011, 03:02 PM

Hi:

Thanks for the tip. This is what nasm did:

Code:

  16 00000000 45                      juan            db      0x45
    17                                  
    18                                  section         .text
    19                                          global _start
    20                                  freq equ 10 % 7
    21                                  len  equ 1532
    22                                  len_low equ len & 0xff
    23                                  
    24                                  _start:
    25 00000000 B003                            mov al,freq
    26 00000002 8025[00000000]03                and byte[juan],freq
    27 00000009 B0FC                            mov al,len & 0xff
    28 0000000B B005                            mov al,(len & 0xff00) / 256
    29 0000000D 90                              nop
    30

By the way, how primitive this assembler is! And yet so popular! Look how it does not list the values of freq, len, len_low when declared. And it does not produce a listing file unless there has been no errors in the assembly.

Sergei Steshenko · 07-10-2011, 05:01 PM

Quote:

Originally Posted by stf92

Hi:

Thanks for the tip. This is what nasm did:

Code:

  16 00000000 45                      juan            db      0x45
    17                                  
    18                                  section         .text
    19                                          global _start
    20                                  freq equ 10 % 7
    21                                  len  equ 1532
    22                                  len_low equ len & 0xff
    23                                  
    24                                  _start:
    25 00000000 B003                            mov al,freq
    26 00000002 8025[00000000]03                and byte[juan],freq
    27 00000009 B0FC                            mov al,len & 0xff
    28 0000000B B005                            mov al,(len & 0xff00) / 256
    29 0000000D 90                              nop
    30

By the way, how primitive this assembler is! And yet so popular! Look how it does not list the values of freq, len, len_low when declared. And it does not produce a listing file unless there has been no errors in the assembly.

How about RTFMing ?

http://www.nasm.us/xdoc/2.09.09/html...#section-2.1.3 :

Quote:

2.1.3 The -l Option: Generating a Listing File

If you supply the -l option to NASM, followed (with the usual optional space) by a file name, NASM will generate a source-listing file for you, in which addresses and generated code are listed on the left, and the actual source code, with expansions of multi-line macros (except those which specifically request no expansion in source listings: see section 4.3.11) on the right. For example:

nasm -f elf myfile.asm -l myfile.lst

If a list file is selected, you may turn off listing for a section of your source with[list -], and turn it back on with[list +], (the default, obviously). There is no "user form" (without the brackets). This can be used to list only sections of interest, avoiding excessively long listings.

Sergei Steshenko · 07-10-2011, 05:04 PM

Code:

(len & 0xff00) / 256

should better be replaced (for clarity) with right non-cyclic shift by 8 bits of 'len'.

stf92 · 07-10-2011, 05:16 PM

I'll try RTFMing, although I picked nasm up because it is the most nearly compatible with MASM I found, and I have a long program written in the MASM assembler that I want to adapt for linux.

About nasm, it sounds incredible, I know, but line 26 of the listing, is not correctly assembled. It says 8025 and it should say 8026. And this translates into the binary file. So either I discard nasm or get a newer version.

But of course I used the -l option. But no listing outpu if there are errors. I wanted to use a shift but I did not know the syntax. Thanks for the post.

Sergei Steshenko · 07-10-2011, 06:47 PM

Quote:

Originally Posted by stf92

I'll try RTFMing, although I picked nasm up because it is the most nearly compatible with MASM I found, and I have a long program written in the MASM assembler that I want to adapt for linux.

About nasm, it sounds incredible, I know, but line 26 of the listing, is not correctly assembled. It says 8025 and it should say 8026. And this translates into the binary file. So either I discard nasm or get a newer version.

But of course I used the -l option. But no listing outpu if there are errors. I wanted to use a shift but I did not know the syntax. Thanks for the post.

http://www.nasm.us/xdoc/2.09.09/html...#section-3.5.4 :

Quote:

3.5.4 << and >>: Bit Shift Operators

<< gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

stf92 · 07-11-2011, 02:37 AM

Thanks for the help. And for the doc references. Do you know a forum dedicated to assembler stuff?

mf93 · 07-11-2011, 06:45 AM

IMO, fasm is a more advanced assembler with nearly identical nasm syntax that is written in assembler, as opposed to nasm being written in c. Also it uses multiple passes to generate a more efficient compilation

stf92 · 07-11-2011, 07:56 AM

That sounds attractive. I'll try to get it. Thanks.

mf93 · 07-12-2011, 09:35 AM

Here is the site: http://flatassembler.net/ it includes download links and documentation.

stf92 · 07-12-2011, 09:40 AM

Thanks. I already downloaded it. It has a drawback: it doesn't output listing files.