I'd previously thought that keeping locals in the innermost scope possible would get a further benefit from overlaying - but someone on the Keil forum recently suggested that this is not so.

Can you comment?


Overlaying works on function scope only. That's because local variable segments are created at the function level but no lower.

In most cases, the compiler re-uses variables in sequential blocks. For example:

line level    source

   1          void main (void)
   2          {
   3   1      // Block 1
   4   1        {
   5   2        unsigned int a, b, c, d, e, f;
   6   2      
   7   2        a = b+c+d+e+f;
   8   2        }
   9   1      
  10   1      // Block 2
  11   1        {
  12   2        unsigned long a, b, c;
  13   2      
  14   2        a = b+c;
  15   2        }
  16   1      
  17   1      // Block 3
  18   1        {
  19   2        unsigned char a, b, c;
  20   2      
  21   2        a = b+c;
  22   2        }
  23   1      }
  24          

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 1
                                           ; SOURCE LINE # 2
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 7
0000 E500        R     MOV     A,b+01H
0002 2500        R     ADD     A,c+01H
0004 CF                XCH     A,R7
0005 E500        R     MOV     A,b
0007 3500        R     ADDC    A,c
0009 CF                XCH     A,R7
000A 2500        R     ADD     A,d+01H
000C CF                XCH     A,R7
000D 3500        R     ADDC    A,d
000F CF                XCH     A,R7
0010 2500        R     ADD     A,e+01H
0012 CF                XCH     A,R7
0013 3500        R     ADDC    A,e
0015 CF                XCH     A,R7
0016 2500        R     ADD     A,f+01H
0018 F500        R     MOV     a+01H,A
001A EF                MOV     A,R7
001B 3500        R     ADDC    A,f
001D F500        R     MOV     a,A
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 14
001F E500        R     MOV     A,b+03H
0021 2500        R     ADD     A,c+03H
0023 F500        R     MOV     a+03H,A
0025 E500        R     MOV     A,b+02H
0027 3500        R     ADDC    A,c+02H
0029 F500        R     MOV     a+02H,A
002B E500        R     MOV     A,b+01H
002D 3500        R     ADDC    A,c+01H
002F F500        R     MOV     a+01H,A
0031 E500        R     MOV     A,b
0033 3500        R     ADDC    A,c
0035 F500        R     MOV     a,A
                                           ; SOURCE LINE # 15
                                           ; SOURCE LINE # 18
                                           ; SOURCE LINE # 21
0037 E500        R     MOV     A,b
0039 2500        R     ADD     A,c
003B F500        R     MOV     a,A
                                           ; SOURCE LINE # 22
                                           ; SOURCE LINE # 23
003D 22                RET     
             ; FUNCTION main (END)



MODULE INFORMATION:   STATIC OVERLAYABLE
   CODE SIZE        =     62    ----
   CONSTANT SIZE    =   ----    ----
   XDATA SIZE       =   ----    ----
   PDATA SIZE       =   ----    ----
   DATA SIZE        =   ----      12
   IDATA SIZE       =   ----    ----
   BIT SIZE         =   ----    ----
END OF MODULE INFORMATION.


C51 COMPILATION COMPLETE.  0 WARNING(S),  0 ERROR(S)

Jon