A) Write the addressing modes used against each instruction.
b) add comments
c) how many time the FIRST, SECOND, and TOP loops run?
d) How loop $ works?
CPU 8086
ORG 0X0400
MOV AL, 1
OUT 0, AL
MOV BX, 0
TOP: MOV CX, 7
FIRST: XCHG BX, CX
MOV CX, 9000H
LOOP $
XCHGBX, CX
SHL AL, 1
OUT 0, AL
LOOP FIRST
MOV CX, 7
SECOND: XCHG BX, CX
MOV CX, 9000H
LOOP $
XCHG BX, CX
SHR AL, 1
OUT 0, AL
LOOP SECOND
JMP TOP
a) Addressing modes used against each instruction:
1) MOV AL, 1 - Immediate addressing mode
2) OUT 0, AL - Direct addressing mode
3) MOV BX, 0 - Immediate addressing mode
4) TOP: - No addressing mode (label)
5) MOV CX, 7 - Immediate addressing mode
6) FIRST: - No addressing mode (label)
7) XCHG BX, CX - Register addressing mode
8) MOV CX, 9000H - Immediate addressing mode
9) LOOP $ - Relative addressing mode
10) XCHG BX, CX - Register addressing mode
11) SHL AL, 1 - Immediate addressing mode
12) OUT 0, AL - Direct addressing mode
13) LOOP FIRST - Relative addressing mode
14) MOV CX, 7 - Immediate addressing mode
15) SECOND: - No addressing mode (label)
16) XCHG BX, CX - Register addressing mode
17) MOV CX, 9000H - Immediate addressing mode
18) LOOP $ - Relative addressing mode
19) XCHG BX, CX - Register addressing mode
20) SHR AL, 1 - Immediate addressing mode
21) OUT 0, AL - Direct addressing mode
22) LOOP SECOND - Relative addressing mode
23) JMP TOP - Direct addressing mode
b) Comments added:
CPU 8086
ORG 0X0400
MOV AL, 1 ; Move immediate value 1 to AL register
OUT 0, AL ; Output the value in AL to port 0
MOV BX, 0 ; Move immediate value 0 to BX register
TOP: ; Label for top loop
MOV CX, 7 ; Move immediate value 7 to CX register
FIRST: ; Label for first loop
XCHG BX, CX ; Exchange the values in BX and CX registers
MOV CX, 9000H ; Move immediate value 9000H to CX register
LOOP $ ; Loop back to FIRST label based on CX register value
XCHG BX, CX ; Exchange the values in BX and CX registers
SHL AL, 1 ; Shift left AL register by 1
OUT 0, AL ; Output the value in AL to port 0
LOOP FIRST ; Loop back to FIRST label based on CX register value
MOV CX, 7 ; Move immediate value 7 to CX register
SECOND: ; Label for second loop
XCHG BX, CX ; Exchange the values in BX and CX registers
MOV CX, 9000H ; Move immediate value 9000H to CX register
LOOP $ ; Loop back to SECOND label based on CX register value
XCHG BX, CX ; Exchange the values in BX and CX registers
SHR AL, 1 ; Shift right AL register by 1
OUT 0, AL ; Output the value in AL to port 0
LOOP SECOND ; Loop back to SECOND label based on CX register value
JMP TOP ; Jump to TOP label
c) The FIRST loop runs 7 times, the SECOND loop also runs 7 times, and the TOP loop runs indefinitely (until a jump or a halt instruction is encountered).
d) The loop $ instruction is a relative addressing mode instruction. It decrements the value in the CX register by 1 and checks its value. If the value in CX is not zero, it loops back to the label marked by the $ symbol (in this case, either FIRST or SECOND). This allows the program to repeat the instructions within the loop a certain number of times.
explain d) in simple terms
In simple terms, the loop $ instruction is used to repeat a set of instructions a specific number of times. The number of times the instructions are repeated is determined by the value in the CX register at the beginning of the loop.
The loop instruction first decreases the value in the CX register by 1. It then checks if the value in CX is zero or not. If it is not zero, the program jumps back to a specific label marked by the $ symbol, which indicates the starting point of the loop. This means that the instructions within the loop are executed again.
This process continues until the value in the CX register becomes zero. Once the value in CX becomes zero, the program moves on to the next instruction outside of the loop. By using the loop $ instruction, the program can repeat a certain set of instructions multiple times without having to write those instructions multiple times in the code.
a) Here are the addressing modes used against each instruction:
MOV AL, 1 - Immediate addressing mode
OUT 0, AL - Direct addressing mode
MOV BX, 0 - Immediate addressing mode
TOP: - No addressing mode used
MOV CX, 7 - Immediate addressing mode
FIRST: - No addressing mode used
XCHG BX, CX - Register addressing mode
MOV CX, 9000H - Immediate addressing mode
LOOP $ - Relative addressing mode
XCHG BX, CX - Register addressing mode
SHL AL, 1 - Immediate addressing mode
OUT 0, AL - Direct addressing mode
LOOP FIRST - Relative addressing mode
MOV CX, 7 - Immediate addressing mode
SECOND: - No addressing mode used
XCHG BX, CX - Register addressing mode
MOV CX, 9000H - Immediate addressing mode
LOOP $ - Relative addressing mode
XCHG BX, CX - Register addressing mode
SHR AL, 1 - Immediate addressing mode
OUT 0, AL - Direct addressing mode
LOOP SECOND - Relative addressing mode
JMP TOP - Relative addressing mode
b) Here are the comments added to each instruction:
CPU 8086
ORG 0X0400
MOV AL, 1 ; Move immediate value 1 to AL register
OUT 0, AL ; Output AL value to port 0
MOV BX, 0 ; Move immediate value 0 to BX register
TOP: MOV CX, 7 ; Move immediate value 7 to CX register
FIRST: XCHG BX, CX ; Exchange the values of BX and CX registers
MOV CX, 9000H ; Move immediate value 9000H to CX register
LOOP $ ; Loop back to the current location based on CX value
XCHG BX, CX ; Exchange the values of BX and CX registers
SHL AL, 1 ; Shift the AL register to the left by 1 bit
OUT 0, AL ; Output AL value to port 0
LOOP FIRST ; Loop back to the FIRST label based on CX value
MOV CX, 7 ; Move immediate value 7 to CX register
SECOND: XCHG BX, CX ; Exchange the values of BX and CX registers
MOV CX, 9000H ; Move immediate value 9000H to CX register
LOOP $ ; Loop back to the current location based on CX value
XCHG BX, CX ; Exchange the values of BX and CX registers
SHR AL, 1 ; Shift the AL register to the right by 1 bit
OUT 0, AL ; Output AL value to port 0
LOOP SECOND ; Loop back to the SECOND label based on CX value
JMP TOP ; Jump to the TOP label
c) The number of times the loops run depend on the initial value of the CX register. However, since the assembly code provided does not specify the initial value for CX, it is not possible to determine the exact number of times the FIRST, SECOND, and TOP loops will run.
d) The "LOOP $" instruction is a relative loop instruction. It decrements the CX register and, if the result is not zero, it causes a jump to the current location (the "$" symbol represents the current location). This creates a loop that repeats until the CX register becomes zero.