文字VS立即操作数 [英] Literals VS Immediate Operands
问题描述
在本学期的系统软件课程中，我们正在学习组装程序和其他系统软件.在阅读本课程时，我遇到了文学主题.
In the systems software course that I have this semester, we are being taught assemblers and other system software. While reading across the course I came across the topic of LITERALS.
文字与立即操作数之间有一个比较，即它们之间的唯一区别是文字不作为指令的一部分进行汇编，而立即操作数则作为指令的一部分.
There was a comparison between literals and immediate operands that said that the only difference between them is that literals are not assembled as a part of the instruction, whereas immediate operands are.
如果可以使用立即数操作数，为什么必须使用文字?是什么让他们与众不同?换句话说，什么时候使用文字，什么时候使用立即数?
推荐答案
立即操作数是可以将其编码为指令本身的文字值，例如
Immediate operands are literal values you can encode into the instructions themselves, e.g.,
MOV R1, 17 ; move 17 as a value to R1
但是，您可能需要将文字值放入程序可能使用的数据结构或表中.您可以使用声明存储的汇编程序"psuedo-ops"来执行此操作:
But, you may have need to put literal values into data structures or tables that your program may use. You use assembler "psuedo-ops" that declare storage to do this:
DW 17 ; define a word containing the literal 17
一些文字，尤其是文本字符串，几乎永远无法放入立即数字段(立即数字段中位数不足)，因此实际上不能作为指令立即值包含在程序中:
Some literals, notably text strings, almost never fit into the immediate field (not enough bits in the immediate field), so really cannot be included in your program as instruction-immediate values:
XYZ DC "I am a duck."
发生这种情况时，通常会发现通过其标签将声明的数据引用为隐式立即值(而不是文字)的指令:
When this happens, you will typically find instructions that refer to the declared data via its label as an implicit immediate value, which is not a literal:
MOV R1, XYZ
一个例外是字符串文字非常短:
An exception is an extremely short string literal:
MOV R1, "A"
这与call语句中的隐式文字并没有什么不同:
This isn't really different than the implicit literal in a call statement:
CALL FOO
指的是FOO代码上的标签:
which refers to the label on the FOO code:
FOO: MOV R1, 17
RETURN
您的汇编器语法可能与此不同，但是原理保持不变.
Your assembler syntax may vary from this, but the principles stay the same.
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