Revision as of 18:49, 16 June 2016

The Original Code

Consider the following C function:

<c> extern int printf(const char *format, ...); int printlist(int lo, int hi) {

 int ix = lo;
 while (ix < hi) {
   printf("%d\n", ix);
   ix++;
 }
 return ix;

} </c>

Postfix Code

The Postfix code for the above function is as follows:

<asm> EXTRN printf

TEXT ALIGN GLOBL printlist, FUNC LABEL printlist

ENTER 4  ; ix@-4 lo@+8 hi@+12

int ix = lo is NOT an assignment

LOCV +8 ; read lo LOCA -4 ; write to ix

ALIGN LABEL whiletest LOCV -4  ; read ix LOCV +12 ; read hi LT JZ whileend

start while body

prepare arguments for calling printf

LOCV -4  ; second printf arg: read ix

put string literal in read-only memory

RODATA ALIGN LABEL strlit STR "%d\n"

now get the address of the string literal (strlit)

TEXT ADDR strlit ; first printf arg: string literal address

args are in the stack

call the function

CALL printf

clean args

TRASH 8

get printf return value

PUSH

get rid of return value (printf used as instruction)

TRASH 4

increment ix

LOCV -4 ; read ix DUP INT 1 ADD LOCA -4 ; ix = ix + 1

trash old value of ix

TRASH 4

end while body

restart while cycle

JMP whiletest

this is the while exit point

ALIGN LABEL whileend

prepare return value

LOCV -4 ; read ix

put it in the accumulator (register) to conform with Cdecl

POP

LEAVE  ; release stack frame RET  ; return control to caller

</asm>

Compiling and Running

To compile the Postfix code directly, pf2asm can be used:

• pf2asm while.pf
• yasm -felf while.asm