Using Assembler in Delphi

Table 1: Use of CPU registers

This table summarises register usage in Delphi 32-bit applications. The first column lists the different CPU registers. The second column shows what the register contains upon entry and the third indicates what the register holds upon exit. The fourth column tells you whether or not you are allowed to use the register inside your own code and the last column indicates if you need to preserve the content of the register (save it upon entry and restore it before leaving).

	Entry	Exit	Use allowed?	Preserve?
eax	Self (1), First parameter (2) or undefined (3)	Function result (4)		No
ebx	Unknown	n/a		Yes
ecx	Second parameter (1), Third parameter (2) or undefined (3)	n/a		No
edx	First parameter (1), Second parameter (2) or undefined (3)	For Int64 result type: highest dword, otherwise n/a		No
esi	Undefined	n/a		Yes
edi	Undefined	n/a		Yes
ebp	Stack frame pointer	Stack frame pointer	(5)	Yes
esp	Stack pointer	Stack pointer	(5)	Yes
cs	Code selector	n/a		Yes
ds	Data selector	n/a		Yes
es	Data selector	n/a		Yes
fs	Thread information block (TIB) selector	n/a		Yes
gs	Reserved	n/a		Yes
ss	Stack selector	Stack selector		Yes

⁽¹⁾ Upon entry of methods when using the Register calling convention.
⁽²⁾ Upon entry of stand-alone functions and procedures while using the Register calling convention.
⁽³⁾ Upon entry for methods and stand-alone functions and procedures in all calling conventions except Register.
⁽⁴⁾ Only for Result types that qualify to go into a register. See table 4 for a complete overview of how results are returned to the caller.
⁽⁵⁾ While you are allowed to use these registers inside your code, such practise is strongly discouraged. The stack pointer itself changes with every stack operation and its content is therefore highly volatile, making it difficult to manage in code. The ebp register is used to access the stack frame and other uses are therefore to be avoided.