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diff --git a/doc/asm.html b/doc/asm.html index ba1970064..b855b9ef7 100644 --- a/doc/asm.html +++ b/doc/asm.html @@ -1,6 +1,6 @@ <!--{ "Title": "A Quick Guide to Go's Assembler", - "Path": "/doc/asm.html" + "Path": "/doc/asm" }--> <h2 id="introduction">A Quick Guide to Go's Assembler</h2> @@ -113,12 +113,30 @@ is the name <code>foo</code> as an address in memory. </p> <p> -The <code>FP</code> is a virtual frame pointer. +The <code>FP</code> pseudo-register is a virtual frame pointer +used to refer to function arguments. The compilers maintain a virtual frame pointer and refer to the arguments on the stack as offsets from that pseudo-register. Thus <code>0(FP)</code> is the first argument to the function, <code>8(FP)</code> is the second (on a 64-bit machine), and so on. -To refer to an argument by name, add the name to the numerical offset, like this: <code>first_arg+0(FP)</code>. -The name in this syntax has no semantic value; think of it as a comment to the reader. +When referring to a function argument this way, it is conventional to place the name +at the beginning, as in <code>first_arg+0(FP)</code> and <code>second_arg+8(FP)</code>. +Some of the assemblers enforce this convention, rejecting plain <code>0(FP)</code> and <code>8(FP)</code>. +For assembly functions with Go prototypes, <code>go vet</code> will check that the argument names +and offsets match. +</p> + +<p> +The <code>SP</code> pseudo-register is a virtual stack pointer +used to refer to frame-local variables and the arguments being +prepared for function calls. +It points to the top of the local stack frame, so references should use negative offsets +in the range [−framesize, 0): +<code>x-8(SP)</code>, <code>y-4(SP)</code>, and so on. +On architectures with a real register named <code>SP</code>, the name prefix distinguishes +references to the virtual stack pointer from references to the architectural <code>SP</code> register. +That is, <code>x-8(SP)</code> and <code>-8(SP)</code> are different memory locations: +the first refers to the virtual stack pointer pseudo-register, while the second refers to the +hardware's <code>SP</code> register. </p> <p> @@ -358,11 +376,26 @@ MOVQ m(CX), BX // Move m into BX. <h3 id="arm">ARM</h3> <p> -The registers <code>R9</code> and <code>R10</code> are reserved by the -compiler and linker to point to the <code>m</code> (machine) and <code>g</code> +The registers <code>R9</code>, <code>R10</code>, and <code>R11</code> +are reserved by the compiler and linker. +</p> + +<p> +<code>R9</code> and <code>R10</code> point to the <code>m</code> (machine) and <code>g</code> (goroutine) structures, respectively. -Within assembler source code, these pointers -can be referred to as simply <code>m</code> and <code>g</code>. +Within assembler source code, these pointers must be referred to as <code>m</code> and <code>g</code>; +the names <code>R9</code> and <code>R10</code> are not recognized. +</p> + +<p> +To make it easier for people and compilers to write assembly, the ARM linker +allows general addressing forms and pseudo-operations like <code>DIV</code> or <code>MOD</code> +that may not be expressible using a single hardware instruction. +It implements these forms as multiple instructions, often using the <code>R11</code> register +to hold temporary values. +Hand-written assembly can use <code>R11</code>, but doing so requires +being sure that the linker is not also using it to implement any of the other +instructions in the function. </p> <p> @@ -370,6 +403,10 @@ When defining a <code>TEXT</code>, specifying frame size <code>$-4</code> tells the linker that this is a leaf function that does not need to save <code>LR</code> on entry. </p> +<p> +The name <code>SP</code> always refers to the virtual stack pointer described earlier. +For the hardware register, use <code>R13</code>. +</p> <h3 id="unsupported_opcodes">Unsupported opcodes</h3> |