Keil C51's 18th keyword extension of C language: using

In the 8051 series microcontrollers, the first 32 bytes of the internal RAM are divided into 4 groups, each with 8 registers. The names of the 8 registers in each group are R0-R7. By setting two bits in the PSW register, you can choose which of the 4 groups of registers to use.

Register sets are very useful when handling interrupts or using real-time operating systems. You can use different register sets when entering an interrupt or switching tasks, without saving the contents of the 8 registers to the stack. When exiting an interrupt or returning to the original task, you only need to switch back to the original register set.

Specify the register bank to be used by a function:

void rb_function (void) using 3
  {
  .
  .
  .
  }

Using is followed by a number 0-3. Symbolic expressions are not allowed to replace the numeric constant. The using keyword can only be used when defining a function. It is not allowed when declaring a function prototype.

A function declared using has the following characteristics:

1) At the function entry, the currently used register set is saved to the stack;

2) The specified register group is set;

3) Before the function exits, the original register set is restored;

The following shows the usage of using. Please pay attention to the codes at the function entry and exit in the generated assembly program:

C program:

   1
   2 external bit alarm;
   3         int alarm_count;
   4         extern void alfunc (bit b0);
   5
   6         void falarm (void) using 3  {
   7   1           alarm_count++;
   8   1           alfunc (alarm = 1);
   9   1         }


Generated assembler:
0000 C0D0 PUSH PSW /* At the function entry, save the original register group to the stack */
0002 75D018 MOV PSW,#018H /* Switch to new register bank */

0005 0500   R   INC   alarm_count+01H
0007 E500   R   MOV   A,alarm_count+01H
0009 7002       JNZ   ?C0002
000B 0500   R   INC   alarm_count
000D ?C0002:
000D D3 SETB C
000E 9200   E   MOV   alarm,C
0010 9200   E   MOV   ?alfunc?BIT,C
0012 120000 E   LCALL alfunc

0015 D0D0 POP PSW /* Before exiting the function, switch to the original register group */
0017 22 COURT

Functions whose return values ​​are stored in registers cannot use using.

Even if the same register set is used, a using function cannot return a bit type value.

Using is usually used when defining interrupt service functions. We can specify different register groups for different interrupt service functions, which can reduce stack operations and improve program running efficiency.