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--- en:multiasm:papc:chapter_6_6 [2025/07/31 12:19] – [MOD R/M byte] ktokarz
+++ en:multiasm:papc:chapter_6_6 [2025/08/01 06:59] (current) – [Scale Index Base byte] ktokarz
@@ Line 308: / Line 308: @@
 In the tables {{ref>SIB_index}} and {{ref>SIB_index64}}, the * means that there is no index register encoded in the instruction. It results in base-only or direct addressing.
-Let's look at some code examples, considering the 32-bit version first.
+Let's look at some code examples, considering the 32-bit version first. In all instructions, there is a MOD R/M byte and an SIB byte. MOD R/M is identical for all instructions. REG field indicates eax register and the combination of MOD and R/M indicates that registers are specified with the SIB byte.
 <code asm>
-mov eax, [ebx+ecx]     ;0x8B, 0x04, 0x0B
+;MOD R/M (second byte) is 0x04 for all instructions:
-mov eax, [ebx+ecx*2]   ;0x8B, 0x04, 0x4B
+                       ;                    MOD REG R/M   REG  MOD & R/M
-mov eax, [ebx+ecx*4]   ;0x8B, 0x04, 0x8B
+                       ;                     00 000 100   eax  SIB is present
-mov eax, [ebx+ecx*8]   ;0x8B, 0x04, 0xCB
+;SIB (third byte) is 0x0B, 0x4B, 0x8B or 0xCB:
+                       ;                  Scale Index Base  Scale Index Base
+mov eax, [ebx+ecx]     ;0x8B, 0x04, 0x0B     00   001  011     x1   ecx  ebx
+mov eax, [ebx+ecx*2]   ;0x8B, 0x04, 0x4B     01   001  011     x2   ecx  ebx
+mov eax, [ebx+ecx*4]   ;0x8B, 0x04, 0x8B     10   001  011     x4   ecx  ebx
+mov eax, [ebx+ecx*8]   ;0x8B, 0x04, 0xCB     11   001  011     x8   ecx  ebx
 </code>
+And other examples for x64 processors. The SIB byte is extended with bits from the REX prefix. We'll start with the similar examples as shown for 32-bit machines.
+<code asm>
+;REX prefix (first byte) is 0x48 for all instructions:
+                       ;                   7                           0
+                       ;                 +---+---+---+---+---+---+---+---+
+                       ;                 | 0   1   0   0 | W | R | X | B |
+                       ;                 +---+---+---+---+---+---+---+---+
+                       ;                                   1   0   0   0
+;MOD R/M (second byte) is 0x04 for all instructions:
+                       ;                    MOD R.REG R/M   REG  MOD & R/M
+                       ;                     00 0.000 100   eax  SIB is present
+                       ;                        Scale X.Index B.Base  Scale Index Base
+mov rax, [rbx+rcx]     ;0x48, 0x8B, 0x04, 0x0B     00   0.001  0.011     x1   rcx  rbx
+mov rax, [rbx+rcx*2]   ;0x48, 0x8B, 0x04, 0x4B     01   0.001  0.011     x2   rcx  rbx
+mov rax, [rbx+rcx*4]   ;0x48, 0x8B, 0x04, 0x8B     10   0.001  0.011     x4   rcx  rbx
+mov rax, [rbx+rcx*8]   ;0x48, 0x8B, 0x04, 0xCB     11   0.001  0.011     x8   rcx  rbx
+</code>
+If any of the new registers (R8-R15) is used in the instruction, it changes the bits in the REX prefix.
+<code asm>
+                       ;                        Scale X.Index B.Base  Scale Index Base
+mov rax, [r10+rcx]     ;0x49, 0x8B, 0x04, 0x0A     00   0.001  1.010     x1   rcx  r10
+mov rax, [rbx+r11]     ;0x4A, 0x8B, 0x04, 0x1B     00   1.001  0.011     x1   r11  rbx
+mov r12, [rbx+rcx]     ;0x4C, 0x8B, 0x24, 0x0B     10   0.001  0.011     x1   rcx  rbx
+                       ;Last instruction has the MOD R/M REG field extended
+                       ;by the R bit from the REX prefix.
+                       ;                    MOD R.REG R/M   REG  MOD & R/M
+                       ;                     00 1.100 100   r12  SIB is present
+</code>
+Certainly, the presented examples do not exhaust all possible situations. For a more detailed explanation, please refer to the documentation by AMD((https://docs.amd.com/v/u/en-US/40332-PUB_4.08)), Intel((https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html)), OSDev wiki((https://wiki.osdev.org/X86-64)) or other interesting sources mentioned at the bottom of this section.
 =====Displacement=====
 Displacement gives the offset for memory operands. Depending on the addressing mode, it can be the direct memory address or an additional offset added to the contents of the base, index register or both. Displacement can be 1, 2, or 4 bytes long. Some instructions allow using an 8-byte displacement. In these instructions, there is no immediate field.

en/multiasm/papc/chapter_6_6.1753964344.txt.gz · Last modified: 2025/07/31 12:19 by ktokarz