《Programming from the Ground Up》学习第3天，p49-p74总结，总计26页。
一、技术总结
1.function
(1)定义
p49, Functions are unit of code that do a defined piece of work on specified types of data。
函数是在指定类型的数据上完成所定义的某个工作的代码单元。
(2)parameter
p49, The data items a function is given to process are called it's parameters。
给予函数处理的数据项称为函数的参数。
(3)interface
p49, The parameter list and the processing expectations of a function(what it is expected to to with the parameters) are called the function's interface。
函数参数列表和预期处理结果称为函数的接口。
2.primitive functions/primitives
p50，However, ultimately there are things that you cannot write functions for which must be provided by the system. Those are called primiteive function(or just primiteives)-they are teh basics which everything else is built off of.
这些由系统提供的函数称为原函数(或原语)。
3.stack
(1)定义
p51,Each computer program that runs uses a region of memory called the stack to enable functions to work properly.
The computer's stack lives at the very top address of memory.
stack(栈)就是memory(内存)的一块区域，位于内存的最顶端。这是物理意义的上的。
4.power.s
#PURPOSE: Program to illustrate how functions work
# This program will compute the value of
# 2^3 + 5^2
#
#Everything in the main program is stored in registers,
#so the data section doesn't have anything
.section .data #.data的作用是什么？
.section .text #.text的作用是什么？
.global _start
_start:
pushq $3 #push second argument
pushq $2 #push first argument
call power # call the function
add $8, %rsp #movqe the stack pointer back
pushq %rax #save the first answer before
#calling the next function
pushq $2 #push second argument
pushq $5
call power #call the function
add $8, %rsp #movqe the stack pointer back
popq %rbx #The second answer is already
#in %rax. We save the
#first answer onto the stack,
#so now we can just pop it
#out into %rbx
add %rax, %rbx #add them together
#the result is in %rbx
movq $1, %rax #exit (%rbx is returned)
int $0x80
#PURPOSE: This function is used to compute
#the value of a number raised to a power.
#
#INPUT: First argument-the base number
#Second argument-the power to raise it to
#
#OUTPUT: Will give the result as a return value
#
#NOTES: The power must be 1 or greater
#
#VARIABLES:
#%rbx - holds the base number
#%rcx - holds the power
#
#-4(%rbp) - holds the current result
#
# %rax is used for temporary storage
#
.type power, @function
power:
pushq %rbp #save old base pointer
movq %rsp, %rbp # make stack pointer the base pointer
sub $4, %rsp #get room for our local storage
movq 8(%rbp), %rbx #put first argument in %rax
movq 12(%rbp), %rcx #put second argument in %rcx
movq %rbx, -4(%rbp) #store current result
power_loop_start:
cmp $1, %rcx #if the power is 1, we are done
je end_power
movq -4(%rbp), %rax #movqe the current result into %rax
imul %rbx, %rax #multiply the current result by
#the base number
movq %rax, -4(%rbp) #store the current result
dec %rcx #decrease the power
jmp power_loop_start # run for the next power
end_power:
movq -4(%rbp), %rax # return value goes in %rax
movq %rbp, %rsp #restore the stack pointer
popq %rbp #restore the base pointer
ret
书里使用的是32位的电脑，但我们现在的电脑大多是64位的，如果运行书中的代码就会提示：Error: invalid instruction suffix for `push'，这里对代码做了修改，改为64位电脑里的寄存器。
二、英语总结
无。
三、其它
1.8、16、32、64位电脑里寄存器的名字
因为不同教材使用的寄存器名称不一样，容易对初学者造成困扰，这里把8位、16位、32位、64位电脑里的寄存器名称全部列出来，这样在看到这些名字的时候我们就知道作者使用的是多少位的电脑。同时我们尽量在对应位数的电脑里去运行这些代码。
r8 = AL AH BL BH CL CH DL DH
r16 = AX BX CX DX BP SP SI DI
r32 = EAX EBX ECX EDX EBP ESP ESI EDI
r64 = RAX RBX RCX RDX RBP RSP RSI RDI R8 R9 R10 R11 R12 R13 R14 R15
2.32和64位电脑里寄存器调用区别
(1)x86 Assembly pushl/popl don't work with "Error: suffix or operands invalid"
(2)What are the calling conventions for UNIX & Linux system calls (and user-space functions) on i386 and x86-64
四、参考资料
1. 编程
(1)Jonathan Bartlett，《Programming From The Ground Up》：https://book.douban.com/subject/1787855/
2. 英语
(1)Etymology Dictionary：https://www.etymonline.com
(2) Cambridge Dictionary：https://dictionary.cambridge.org
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