先决条件–寻址模式
1.直接寻址模式：
在直接寻址模式下，地址字段包含操作数的地址。
Effective Address (EA) = address field of operand 
示例：将寄存器1001的内容添加到累加器。
Add (1001) 只需一个内存引用即可访问数据。因此，无需其他计算即可得出有效地址。
2.立即寻址模式：
在立即寻址模式下，操作数是指令的一部分。
Here the Operand = Address Field 
示例：将5加到累加器上。
ADD 5 不需要内存引用即可获取数据。因此，无需其他计算即可得出有效地址。这是一种快速的方法。但是缺点是范围有限。
现在让我们比较直接和立即寻址模式。
| Direct Addressing Mode | Immediate Addressing Mode |
|---|---|
| Address fields contains the effective address of operand | There is no address field as the operand is a part of the instruction. |
| It requires one reference to memory. | It does not require any reference to memory. |
| It is slower compared to immediate mode. | It is a faster process. |
| It has more range than in immediate mode. | It has a limited range. |
| Example: Add (1001) | Example: ADD 5 |