4.51 Res Directive

The MPASM RES directive reserves memory by advancing the location counter.

Suggested Replacement

The PIC Assembler's DS directive has a similar function, but there are some differences in its operation.

The DS directive advances the location counter, allowing memory to be allocated to a label defined before the directive. Typically it is used inside psects linked to the data space, providing a mechanism to reserve memory for variables, as in the following example.

PSECT udata_bank0
input:
  DS 2  ;allocate 2 bytes for input
output:
  DS 4  ;allocate 4 bytes for output

If the directive is used in a psect assigned to program memory (space=0 flag), it will move the location counter, but not place anything in the HEX file output.

The address units of this directive are determined by context, in particular, the delta and bit flag values associated with the psect in which it resides. If the delta flag is set to 1, then the directive moves the location counter forward by the specified number of bytes; if it is 2, it moves it forward by 16-bit words, etc. When the bit flag is used, the allocation units are bits.