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| <h4 class="subsection">9.20.2 M32R Directives</h4> |
| |
| <p><a name="index-directives_002c-M32R-1127"></a><a name="index-M32R-directives-1128"></a> |
| The Renease M32R version of <code>as</code> has a few architecture |
| specific directives: |
| |
| |
| <a name="index-g_t_0040code_007blow_007d-directive_002c-M32R-1129"></a> |
| <dl><dt><code>low </code><var>expression</var><dd>The <code>low</code> 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: |
| |
| <pre class="smallexample"> or3 r0, r0, #low(0x12345678) ; compute r0 = r0 | 0x5678 |
| add3, r0, r0, #low(fred) ; compute r0 = r0 + low 16-bits of address of fred |
| </pre> |
| <br><dt><code>high </code><var>expression</var><dd><a name="index-g_t_0040code_007bhigh_007d-directive_002c-M32R-1130"></a>The <code>high</code> 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: |
| |
| <pre class="smallexample"> seth r0, #high(0x12345678) ; compute r0 = 0x12340000 |
| seth, r0, #high(fred) ; compute r0 = upper 16-bits of address of fred |
| </pre> |
| <br><dt><code>shigh </code><var>expression</var><dd><a name="index-g_t_0040code_007bshigh_007d-directive_002c-M32R-1131"></a>The <code>shigh</code> directive is very similar to the <code>high</code> |
| 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 <code>shigh</code> 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 <code>shigh</code> directive pre-biases the upper |
| 16 bit value by adding one to it. For example: |
| |
| <p>For example: |
| |
| <pre class="smallexample"> seth r0, #shigh(0x12345678) ; compute r0 = 0x12340000 |
| seth r0, #shigh(0x00008000) ; compute r0 = 0x00010000 |
| </pre> |
| <p>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. |
| |
| <p>This behaviour is to allow for the different semantics of the |
| <code>or3</code> and <code>add3</code> instructions. The <code>or3</code> instruction |
| treats its 16-bit immediate argument as unsigned whereas the |
| <code>add3</code> treats its 16-bit immediate as a signed value. So for |
| example: |
| |
| <pre class="smallexample"> seth r0, #shigh(0x00008000) |
| add3 r0, r0, #low(0x00008000) |
| </pre> |
| <p>Produces the correct result in r0, whereas: |
| |
| <pre class="smallexample"> seth r0, #shigh(0x00008000) |
| or3 r0, r0, #low(0x00008000) |
| </pre> |
| <p>Stores 0xffff8000 into r0. |
| |
| <p>Note - the <code>shigh</code> 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 <code>or3</code> instruction is being used rather |
| than an <code>add3</code> instruction. It is up to the programmer to make |
| sure that correct directives are used. |
| |
| <p><a name="index-g_t_0040code_007b_002em32r_007d-directive_002c-M32R-1132"></a><br><dt><code>.m32r</code><dd>The directive performs a similar thing as the <em>-m32r</em> command |
| line option. It tells the assembler to only accept M32R instructions |
| from now on. An instructions from later M32R architectures are |
| refused. |
| |
| <p><a name="index-g_t_0040code_007b_002em32rx_007d-directive_002c-M32RX-1133"></a><br><dt><code>.m32rx</code><dd>The directive performs a similar thing as the <em>-m32rx</em> command |
| line option. It tells the assembler to start accepting the extra |
| instructions in the M32RX ISA as well as the ordinary M32R ISA. |
| |
| <p><a name="index-g_t_0040code_007b_002em32r2_007d-directive_002c-M32R2-1134"></a><br><dt><code>.m32r2</code><dd>The directive performs a similar thing as the <em>-m32r2</em> command |
| line option. It tells the assembler to start accepting the extra |
| instructions in the M32R2 ISA as well as the ordinary M32R ISA. |
| |
| <p><a name="index-g_t_0040code_007b_002elittle_007d-directive_002c-M32RX-1135"></a><br><dt><code>.little</code><dd>The directive performs a similar thing as the <em>-little</em> 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. |
| |
| <p><a name="index-g_t_0040code_007b_002ebig_007d-directive_002c-M32RX-1136"></a><br><dt><code>.big</code><dd>The directive performs a similar thing as the <em>-big</em> 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. |
| |
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