一下为3个汇编语言的问题，其中小菜几点疑问，请教，谢谢

Assume the BC register value is 8538H, and the DE register　value is 62A5H.Find the value of register BC after the following　assembly operations:

MOV A,C
SUB E
MOV C,A
MOV A,B
SBB D
MOV B,A
此题中的SUB，SBB 的用法是否错误，资料中很多运算后面有两个寄存器或者数字，为何，这个为一个！谢谢
In the Assembly code shown below
LOOP: MVI C,78H
DCR C
JNZ LOOP
HLT
How many times is the DCR C Operation executed?
此题中的MVI和MOV的区别是啥，谢谢
Describe the most efficient way(in term of execution time　and code size) to divide a number by 4 in assembly language
简单的小菜明白，他说的最优是啥意思？
what value is stored in m in the following assembly language code　fragment if n=7?
LDAA #n
LABEL1: CMPA #5
BHI L3
BEQ L2
DECA
BRA L1
LABEL2: CLRA
LABEL3: STAA #m
这些汇编指令哪位大侠知道是那种类型的。谢谢