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doc: add s390x information to asm.html
Fixes #16362 Change-Id: I676718a1149ed2f3ff80cb031e25de7043805399 Reviewed-on: https://go-review.googlesource.com/25157 Reviewed-by: Rob Pike <r@golang.org>
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@ -780,6 +780,64 @@ mode as on the x86, but the only scale allowed is <code>1</code>.
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</ul>
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<h3 id="s390x">IBM z/Architecture, a.k.a. s390x</h3>
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<p>
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The registers <code>R10</code> and <code>R11</code> are reserved.
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The assembler uses them to hold temporary values when assembling some instructions.
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</p>
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<p>
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<code>R13</code> points to the <code>g</code> (goroutine) structure.
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This register must be referred to as <code>g</code>; the name <code>R13</code> is not recognized.
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</p>
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<p>
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<code>R15</code> points to the stack frame and should typically only be accessed using the
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virtual registers <code>SP</code> and <code>FP</code>.
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</p>
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<p>
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Load- and store-multiple instructions operate on a range of registers.
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The range of registers is specified by a start register and an end register.
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For example, <code>LMG</code> <code>(R9),</code> <code>R5,</code> <code>R7</code> would load
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<code>R5</code>, <code>R6</code> and <code>R7</code> with the 64-bit values at
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<code>0(R9)</code>, <code>8(R9)</code> and <code>16(R9)</code> respectively.
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</p>
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<p>
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Storage-and-storage instructions such as <code>MVC</code> and <code>XC</code> are written
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with the length as the first argument.
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For example, <code>XC</code> <code>$8,</code> <code>(R9),</code> <code>(R9)</code> would clear
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eight bytes at the address specified in <code>R9</code>.
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</p>
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<p>
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If a vector instruction takes a length or an index as an argument then it will be the
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first argument.
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For example, <code>VLEIF</code> <code>$1,</code> <code>$16,</code> <code>V2</code> will load
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the value sixteen into index one of <code>V2</code>.
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Care should be taken when using vector instructions to ensure that they are available at
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runtime.
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To use vector instructions a machine must have both the vector facility (bit 129 in the
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facility list) and kernel support.
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Without kernel support a vector instruction will have no effect (it will be equivalent
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to a <code>NOP</code> instruction).
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</p>
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<p>
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Addressing modes:
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</p>
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<ul>
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<li>
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<code>(R5)(R6*1)</code>: The location at <code>R5</code> plus <code>R6</code>.
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It is a scaled mode as on the x86, but the only scale allowed is <code>1</code>.
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</li>
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</ul>
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<h3 id="unsupported_opcodes">Unsupported opcodes</h3>
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<p>
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