80x86 16-bit asm: lea cx, [cx*8+cx] causes error on NASM (compiling .com file)

Question

The error NASM gives (despite my working OS) is "invalid effective address".

Now I've seen many examples of how to use LEA and I think I got it right but yet my NASM dislikes it. I tried lea cx, [cx+9] and it worked; lea cx, [bx+cx] didn't.

Now if I extended my registers to 32-bits (i.e. lea ecx, [ecx*8+ecx]) everything would be well but i am restricted to use 16- and 8-bit registers only.

Is here anyone so knowledgeable who could explain me WHY my assembler doesn't let me use lea the way I supposed it should be used?

Hydnocarpate

I think you are mistaken when you say that lea cx, [cx+9] worked. – Oldline

Answer 1

This is because [bx+cx] isn't valid in any addressing mode on 16-bit x86, see this site for more info.

lea cx, [bx+di] or lea cx, [bx+si] should work.

If your code will run on 386 or later in 16bit mode, you can use lea cx, [ecx + 9] (address-size prefix but still 16bit operand-size).

See also this Q&A on x86 addressing modes (mostly discussing 32/64bit addressing modes, and the x86 tag wiki.

Euclid

Answer 2

lea cx,[cx*8+cx] doesn't work because "scale-index-base" addressing is only available with 32-bit registers. It's not a limitation of the assembler--it's a limitation of the processor.

Oldline

Answer 3

These following tables for to build a postbyte shows which register can be used as an addressregister and which of them can be combine with a second addressregister(baseregister+indexregister and maybe scaling) to build an address with it. (Observed from the 16 bit address mode, where the D-flag is not set.)

Instruction Prefix                0 or 1 Byte
Address-Size Prefix               0 or 1 Byte
Operand-Size Prefix               0 or 1 Byte
Segment Prefix                    0 or 1 Byte
Opcode                            1 or 2 Byte
Mod R/M (Postbyte)                0 or 1 Byte
SIB, Scale Index Base (386+)      0 or 1 Byte
Displacement                      0, 1, 2 or 4 Byte (4 only 386+)
Immediate                         0, 1, 2 or 4 Byte (4 only 386+)

Format of Postbyte(Mod R/M from Intel-Manual)
------------------------------------------
MM RRR MMM

MM  - Memeory addressing mode
RRR - Register operand address
MMM - Memoy operand address

RRR Register Names
Filds  8bit  16bit  32bit
000    AL     AX     EAX
001    CL     CX     ECX
010    DL     DX     EDX
011    Bl     BX     EBX
100    AH     SP     ESP
101    CH     BP     EBP
110    DH     SI     ESI
111    BH     DI     EDI

---

16bit memory (No 32 bit memory address prefix)
MMM   Default MM Field
Field Sreg     00        01          10             11=MMM is reg
000   DS       [BX+SI]   [BX+SI+o8]  [BX+SI+o16]
001   DS       [BX+DI]   [BX+DI+o8]  [BX+DI+o16]
010   SS       [BP+SI]   [BP+SI+o8]  [BP+SI+o16]
011   SS       [BP+DI]   [BP+DI+o8]  [BP+DI+o16]
100   DS       [SI]      [SI+o8]     [SI+o16]
101   DS       [DI]      [DI+o8]     [SI+o16]
110   SS       [o16]     [BP+o8]     [BP+o16]
111   DS       [BX]      [BX+o8]     [BX+o16]
Note: MMM=110,MM=0 Default Sreg is DS !!!!

32bit memory (Has 67h 32 bit memory address prefix)
MMM   Default MM Field
Field Sreg     00        01          10             11=MMM is reg
000   DS       [EAX]     [EAX+o8]    [EAX+o32]
001   DS       [ECX]     [ECX+o8]    [ECX+o32]
010   DS       [EDX]     [EDX+o8]    [EDX+o32]
011   DS       [EBX]     [EBX+o8]    [EBX+o32]
100   SIB      [SIB]     [SIB+o8]    [SIB+o32]
101   SS       [o32]     [EBP+o8]    [EBP+o32]
110   DS       [ESI]     [ESI+o8]    [ESI+o32]
111   DS       [EDI]     [EDI+o8]    [EDI+o32]
Note: MMM=110,MM=0 Default Sreg is DS !!!!

---

SIB is (Scale/Base/Index)
SS BBB III
Note: SIB address calculated as:
<sib address>=<Base>+<Index>*(2^(Scale))

Fild   Default Base
BBB    Sreg    Register   Note
000    DS      EAX
001    DS      ECX
010    DS      EDX
011    DS      EBX
100    SS      ESP
101    DS      o32        if MM=00 (Postbyte)
SS     EBP        if MM<>00 (Postbyte)
110    SS      ESI
111    DS      EDI

Fild  Index
III   register   Note
000   EAX
001   ECX
010   EDX
011   EBX
100              never Index SS can be 00
101   EBP
110   ESI
111   EDI

Fild Scale coefficient
SS   =2^(SS)
00   1
01   2
10   4
11   8

Dirk

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