Chapter 25. M32R Dependent Features

147

 Wuh

This is a shorter synonym for the  warn unmatched high option.

 Wnuh

This is a shorter synonym for the  no warn unmatched high option.

25.2. 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 ex 

pression 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.