Can anybody tell me what the ORG does in TASM? i know the ORG 100h in ".COM" files and why we put it there (PSP) but i saw this code the other day which was something like this:
Posted on 2006-08-25 03:09:42 by XCHGORG is used to set the assembler's location counter (LC). This LC variable is incremented by the size of the opcode during each translation. So ORG 100h simply sets the LC variable to 256 (the number of bytes from the beginning of the .com file that the entry point is located).
In NASM and MASM (and probably TASM), the $ signifies the current LC value. So $-2 would be the current LC value minus 2 bytes. I'm not exactly sure about TASM, but if you do this code in NASM it would overwrite the INT 21h instruction with the instruction that follows, as though that instruction doesn't exist. This is because the INT 21h instruction is 2 bytes long (0CD21h). Although there might be a different meaning for $ in TASM, so I'll wait for someone to confirm or disprove me.
Regards,
Bryant Keller
In NASM and MASM (and probably TASM), the $ signifies the current LC value. So $-2 would be the current LC value minus 2 bytes. I'm not exactly sure about TASM, but if you do this code in NASM it would overwrite the INT 21h instruction with the instruction that follows, as though that instruction doesn't exist. This is because the INT 21h instruction is 2 bytes long (0CD21h). Although there might be a different meaning for $ in TASM, so I'll wait for someone to confirm or disprove me.
Regards,
Bryant Keller
You're right, Synfire.
BTW: XCHG, I hope you're using at least TASM5.0 (version 5.2 supports pentium instructions along with MMX). Earlier TASMs are severly bugged. Yet it is a great assembler with very nice (IMHO) OOP suuport :)
BTW: XCHG, I hope you're using at least TASM5.0 (version 5.2 supports pentium instructions along with MMX). Earlier TASMs are severly bugged. Yet it is a great assembler with very nice (IMHO) OOP suuport :)
P.S:
This code vaild for tasm/masm
Org 123
bla bla bla
Org 456
more bla bla bla
But It's invalid for Nasm
This code vaild for tasm/masm
Org 123
bla bla bla
Org 456
more bla bla bla
But It's invalid for Nasm
P.S:
This code vaild for tasm/masm
Org 123
bla bla bla
Org 456
more bla bla bla
But It's invalid for Nasm
The second ORG will just be ignored, ORG is a one-shot deal in NASM as you are supposed to use it at the beginning of a source file to define the base address of the program.
P.S:
This code vaild for tasm/masm
Org 123
bla bla bla
Org 456
more bla bla bla
But It's invalid for Nasm
The second ORG will just be ignored, ORG is a one-shot deal in NASM as you are supposed to use it at the beginning of a source file to define the base address of the program.
If NASM only lets you use it once, how do you accomplish the same functionality?
Although it's not very often that it's useful to use org repeatedly in average programs, my OS code needs that capability. I use MASM, and it does work, but it'd be nice if I could use NASM too.
http://nasm.sourceforge.net/doc/html/nasmdo10.html
10.1.3 ORG Doesn't Work
People writing boot sector programs in the bin format often complain that ORG doesn't work the way they'd like: in order to place the 0xAA55 signature word at the end of a 512-byte boot sector, people who are used to MASM tend to code
ORG 0
; some boot sector code
ORG 510
DW 0xAA55
This is not the intended use of the ORG directive in NASM, and will not work. The correct way to solve this problem in NASM is to use the TIMES directive, like this:
ORG 0
; some boot sector code
TIMES 510-($-$$) DB 0
DW 0xAA55
The TIMES directive will insert exactly enough zero bytes into the output to move the assembly point up to 510. This method also has the advantage that if you accidentally fill your boot sector too full, NASM will catch the problem at assembly time and report it, so you won't end up with a boot sector that you have to disassemble to find out what's wrong with it.
10.1.3 ORG Doesn't Work
People writing boot sector programs in the bin format often complain that ORG doesn't work the way they'd like: in order to place the 0xAA55 signature word at the end of a 512-byte boot sector, people who are used to MASM tend to code
ORG 0
; some boot sector code
ORG 510
DW 0xAA55
This is not the intended use of the ORG directive in NASM, and will not work. The correct way to solve this problem in NASM is to use the TIMES directive, like this:
ORG 0
; some boot sector code
TIMES 510-($-$$) DB 0
DW 0xAA55
The TIMES directive will insert exactly enough zero bytes into the output to move the assembly point up to 510. This method also has the advantage that if you accidentally fill your boot sector too full, NASM will catch the problem at assembly time and report it, so you won't end up with a boot sector that you have to disassemble to find out what's wrong with it.
Oh okay so this is what i wrote:
So i guess i actually saved a few bytes for jumping to the end of the procedure which is now replaced with only one near jump.
Thank you guys.
.DATA
String1 DB 'Assembly', 0
; ------------------------------
.CODE
WriteStr PROC
PUSH AX
PUSH BX
PUSH BP
MOV BP , SP
MOV BX ,WORD PTR
MOV AH , 0Eh
OR AX , AX
ORG $-2
@@__WriteStrLoop:
DW 10CDh
MOV AL , BYTE PTR
INC BX
TEST AL , AL
JNE @@__WriteStrLoop
POP BP
POP BX
POP AX
RET
WriteStr ENDP
; ------------------------------
START:
PUSH OFFSET String1
CALL WriteStr
ADD SP , 0002hSo i guess i actually saved a few bytes for jumping to the end of the procedure which is now replaced with only one near jump.
Thank you guys.