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[[Category:Lua]]
{{CodeBox|'''function''' registerAssembler(''callback'') ''':''' void}}

Registers a callback function that is called when Cheat Engine's single-line assembler is invoked to convert an instruction into a list of bytes.

The callback can return a byte table to provide the assembled instruction bytes. Return nil to let another registered assembler callback, or the original x86 assembler, handle the instruction.

===Function Parameters===
{|width="85%" cellpadding="10%" cellspacing="0" border="0"
!align="left"|Parameter
!align="left"|Type
!style="width: 80%;background-color:white;" align="left"|Description
|-
|callback
|Function
|The assembler callback to register. It receives the target address and instruction text.
|}

===Returns===
This function does not return a value.

===Callback Parameters===
{|width="85%" cellpadding="10%" cellspacing="0" border="0"
!align="left"|Parameter
!align="left"|Type
!style="width: 80%;background-color:white;" align="left"|Description
|-
|address
|Integer
|The address where the instruction is being assembled.
|-
|instruction
|String
|The single-line assembler instruction text.
|}

===Callback Return Value===
{|width="85%" cellpadding="10%" cellspacing="0" border="0"
!align="left"|Return Type
!style="width: 80%;background-color:white;" align="left"|Description
|-
|Table
|Return a byte table containing the exact bytes to emit for the instruction.
|-
|nil
|Return nil to let another assembler callback or the original x86 assembler handle the instruction.
|}

===Byte Table Format===
The returned byte table should contain integer byte values.

<syntaxhighlight lang="lua" line highlight="1">
return {0x90}
</syntaxhighlight>

This example returns a single NOP byte.

===Examples===

====Register an assembler callback that lets CE handle everything====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
return nil
end)
</syntaxhighlight>

====Assemble a custom single-byte instruction====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
if instruction == "customnop" then
return {0x90}
end

return nil
end)
</syntaxhighlight>

====Assemble a custom two-byte NOP====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
if instruction == "customnop2" then
return {0x66, 0x90}
end

return nil
end)
</syntaxhighlight>

====Normalize the instruction text before comparing====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
local text = instruction:lower()
text = text:gsub("^%s+", "")
text = text:gsub("%s+$", "")

if text == "customnop" then
return {0x90}
end

return nil
end)
</syntaxhighlight>

====Use the address parameter====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
local text = instruction:lower()

if text == "addresslowbyte" then
local lowByte = address & 0xff
return {lowByte}
end

return nil
end)
</syntaxhighlight>

====Support a small custom instruction alias====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
local text = instruction:lower():gsub("^%s+", ""):gsub("%s+$", "")

if text == "ret0" then
-- xor eax,eax
-- ret
return {0x31, 0xC0, 0xC3}
end

return nil
end)
</syntaxhighlight>

====Use with auto assembler====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
if instruction:lower() == "customnop" then
return {0x90}
end

return nil
end)

autoAssemble([[
customnop
]])
</syntaxhighlight>

====Log unhandled instructions====
<syntaxhighlight lang="lua" line highlight="1">
registerAssembler(function(address, instruction)
print(string.format("Assembling at %X: %s", address, instruction))

return nil
end)
</syntaxhighlight>

{{LuaSeeAlso}}

{{Assembly}}

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