9.19.2 M32R Directives

The Renease M32R version of as has a few architecture specific directives:

low expression

The low directive computes the value of its expression and places the lower 16-bits of the result into the immediate-field of the instruction. For example:

             or3   r0, r0, #low(0x12345678) ; compute r0 = r0 | 0x5678
             add3, r0, r0, #low(fred)   ; compute r0 = r0 + low 16-bits of address of fred
     

high expression

The high directive computes the value of its expression and places the upper 16-bits of the result into the immediate-field of the instruction. For example:

             seth  r0, #high(0x12345678) ; compute r0 = 0x12340000
             seth, r0, #high(fred)       ; compute r0 = upper 16-bits of address of fred
     

shigh expression

The shigh directive is very similar to the high directive. It also computes the value of its expression and places the upper 16-bits of the result into the immediate-field of the instruction. The difference is that shigh also checks to see if the lower 16-bits could be interpreted as a signed number, and if so it assumes that a borrow will occur from the upper-16 bits. To compensate for this the shigh directive pre-biases the upper 16 bit value by adding one to it. For example:

For example:

             seth  r0, #shigh(0x12345678) ; compute r0 = 0x12340000
             seth  r0, #shigh(0x00008000) ; compute r0 = 0x00010000
     

In the second example the lower 16-bits are 0x8000. If these are treated as a signed value and sign extended to 32-bits then the value becomes 0xffff8000. If this value is then added to 0x00010000 then the result is 0x00008000.

This behaviour is to allow for the different semantics of the or3 and add3 instructions. The or3 instruction treats its 16-bit immediate argument as unsigned whereas the add3 treats its 16-bit immediate as a signed value. So for example:

             seth  r0, #shigh(0x00008000)
             add3  r0, r0, #low(0x00008000)
     

Produces the correct result in r0, whereas:

             seth  r0, #shigh(0x00008000)
             or3   r0, r0, #low(0x00008000)
     

Stores 0xffff8000 into r0.

Note - the shigh directive does not know where in the assembly source code the lower 16-bits of the value are going set, so it cannot check to make sure that an or3 instruction is being used rather than an add3 instruction. It is up to the programmer to make sure that correct directives are used.

.m32r

The directive performs a similar thing as the -m32r command line option. It tells the assembler to only accept M32R instructions from now on. An instructions from later M32R architectures are refused.

.m32rx

The directive performs a similar thing as the -m32rx command line option. It tells the assembler to start accepting the extra instructions in the M32RX ISA as well as the ordinary M32R ISA.

.m32r2

The directive performs a similar thing as the -m32r2 command line option. It tells the assembler to start accepting the extra instructions in the M32R2 ISA as well as the ordinary M32R ISA.

.little

The directive performs a similar thing as the -little command line option. It tells the assembler to start producing little-endian code and data. This option should be used with care as producing mixed-endian binary files is fraught with danger.

.big

The directive performs a similar thing as the -big command line option. It tells the assembler to start producing big-endian code and data. This option should be used with care as producing mixed-endian binary files is fraught with danger.