为了区分Plan9汇编和Go Plan9 Assembler，下面所有的描述都会改为 Go Assembler.
Go Assembler中的虚拟寄存器：
FP(Frame pointer): arguments and locals. PC(Program counter): jumps and branches. SB(Static base pointer): global symbols. SP(Stack pointer): top of stack.
// 所有用户空间的数据都可以通过FP(局部数据)或SB(全局数据)访问 All user-defined symbols are written as offsets to the pseudo-registers FP (arguments and locals) and SB (globals).
// SB抽象了内存空间，foo(SB)的意思是用foo来代表内存里面的一个地址（赋值？） The SB pseudo-register can be thought of as the origin of memory, so the symbol foo(SB) is the name foo as an address in memory. // foo(SB)可以用来定义全局的function和数据 This form is used to name global functions and data. // foo<>(SB)和C里面的static一个效果，声明只在当前源码文件中可见 Adding <> to the name, as in foo<>(SB), makes the name visible only in the current source file, like a top-level static declaration in a C file. // 可以在引用上加上偏移量，如前面说的foo(SB)， // 如果foo+4(SB)的意思是foo + 4 bytes的地址 Adding an offset to the name refers to that offset from the symbol's address, so foo+4(SB) is four bytes past the start of foo.
// FP用来访问函数的参数 The FP pseudo-register is a virtual frame pointer used to refer to function arguments. // 编译器维护了栈上的参数指针 The compilers maintain a virtual frame pointer and refer to the arguments on the stack as offsets from that pseudo-register. // 0(FP)就是function的第一个参数 Thus 0(FP) is the first argument to the function, // 64位系统上8(FP)就是第二个参数，后面加上偏移量就可以访问更多的参数 8(FP) is the second (on a 64-bit machine), and so on. // 要访问具体function的参数，需要加上name，比如foo+0(FP)获取foo的第一个参数 // foo+8(FP)获取第二个参数 However, when referring to a function argument this way, it is necessary to place a name at the beginning, as in first_arg+0(FP) and second_arg+8(FP). // 编译器中强制要求必须用name来访问FP The assembler enforces this convention, rejecting plain 0(FP) and 8(FP).
// SP是栈指针，和NASM中的RSP类似 The SP pseudo-register is a virtual stack pointer used to refer to frame-local variables and the arguments being prepared for function calls. // SP指向当前local stack frame的栈顶，使用时foo-8(SP)代表foo的栈第8byte It points to the top of the local stack frame, so references should use negative offsets in the range [−framesize, 0): x-8(SP), y-4(SP), and so on.]
// 如果硬件支持SP寄存器，那么不加name的时候就是访问硬件寄存器. // 因此 x-8(SP)和-8(SP)访问的会是不同的内存空间 On architectures with a hardware register named SP, the name prefix distinguishes references to the virtual stack pointer from references to the architectural SP register. That is, x-8(SP) and -8(SP) are different memory locations: the first refers to the virtual stack pointer pseudo-register, while the second refers to the hardware's SP register. // 对SP和PC的访问都应该带上name，如果要访问对应的硬件寄存器， // 可以用RSP On machines where SP and PC are traditionally aliases for a physical, numbered register, in the Go assembler the names SP and PC are still treated specially; for instance, references to SP require a symbol, much like FP. To access the actual hardware register use the true R name. For example, on the ARM architecture the hardware SP and PC are accessible as R13 and R15.
// 用虚拟指令来封装了text/data section The assembler uses various directives to bind text and data to symbol names.
下面是一个真实的Go Plan9汇编中的function定义：
TEXT runtime·profileloop(SB),NOSPLIT,$8 MOVQ $runtime·profileloop1(SB), CX MOVQ CX, 0(SP) CALL runtime·externalthreadhandler(SB) RET
// 定义参数通常需要指定size，如$24-8 the frame size is followed by an argument size, separated by a minus sign. // 如果NOSPLIT没有指定，参数的size是必须指定的 If NOSPLIT is not specified for the TEXT, the argument size must be provided. // Go汇编还可以声明调用.go声明的代码，但需要通过·来声明 // package·function的调用，这里的·可以通过Shift + Option + 9输入.
// Global指定后面跟着DATA指令可以定义全局标签， // 没有赋值的label全部被清零. Global data symbols are defined by a sequence of initializing DATA directives followed by a GLOBL directive. Each DATA directive initializes a section of the corresponding memory. The memory not explicitly initialized is zeroed. The general form of the DATA directive is:
DATA symbol+offset(SB)/width, value
// 上面的语句初始化symbol+offset(SB)的数据中width bytes，赋值为value which initializes the symbol memory at the given offset and width with the // 相对于栈操作，SB的操作都是增地址，栈是减地址 given value. The DATA directives for a given symbol must be written with increasing offsets.
例3: 一个简单的Go程序，通过go tool compile -S 获取Plan9反汇编结果：
$ GOOS=darwin GOARCH=amd64 go tool compile -S examples/e3/e3.go
