國立臺北大學資工系江宥旻

Machine Language Coding and the DEBUG

Converting assembly language instruction to machine code

Machine code instructions of the 8088 vary in the number of bytes used to encode them

• Single byte instruction
• Double bytes instruction and above
• General instruction format

1. Byte1
   • Opcode field
     • 6 bits, specifies the operation
   • Register direction bit(D bit) 看REG放甚麼
     • 0
       $\to$ 來源
     • 1
       $\to$ 目的
   • Data size bit(W bit)
     • 0
       $\to$ 8 bits
     • 1
       $\to$ 16 bits
2. Byte2
   • Mod field(MOD)
     • addressing mode
   • Register field(REG)
   • Register/Memory field(R/M)

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Example 1

Register Addressing Mode

MOV BL, AL

REG	R/M
AL	BL
BL	AL

因為REG和R/M都可以放暫存器，所以AL和BL可以互換位置，答案就會有所不一樣。

• 6-bit opcode for the MOV is 100010
• Encode AL in the REG field of byte 2, D = 0
• Encode AL in the R/M field of byte 2, D = 1
• Byte operation, W = 0
• BYTE 1 = 10001000
• BYTE 1 = 10001010
• REG = AL = 000
• REG = BL = 011
• Second operand is also regiser, MOD = 11
• Destination register is BL, R/M = 011
• Destination register is AL, R/M = 000
• BYTE 2 = 11000011
• BYTE 2 = 11011000
• MOV BL, AL is 88C3h
• MOV BL, AL is 8AD8h

反白是AL和BL交換放的答案

Example 2

Register Inderict Addressing Mode

ADD AX, [SI]

$DS\ast 16+SI$

• Add the 16-bit contents of the memory location indirectly specified by SI to the contents of AX
• 6-bit opcode for the ADD is 000000
• Encode AX in the REG field of byte 2, D = 1
• Word operation, W = 1
• BYTE 1 = 00000011
• REG = AX = 000
• Second operand is in memory, MOD = 00
• Effective address, R/M = 100
• BYTE 2 = 00000100
• ADD AX, [SI] is 0304h

Example 3

Direct Addressing Mode

XOR CL, [1234H]

$DS\ast 16+1234H$

因為CL只有1 byte，所以來源只能是1 byte

• Exclusive-OR the byte of data at memory address 1234h with the byte contents of CL
• 6-bit opcode for the XOR is 001100
• Encode CL in the REG field of byte 2, D = 1
• byte operation, W = 0
• BYTE 1 = 00110010
• REG = CL = 001
• Second operand is in memory, MOD = 00
• Effective address, R/M = 110
• BYTE 2 = 00001110
• BYTE 3 = 34H
• BYTE 4 = 12H
• XOR CL, [1234H] is 320E3412H (Little Endian)

Example 4

Based-Indexed Addressing Mode

ADD [BX][DI] + 1234H, AX

$DX\ast 16+BX+DI+1234H$

• Add the word contents of AX to the contents of the memory location specified by based-indexed address mode
• 6-bit opcode for the ADD is 000000
• AX is at source, Encode AX in the REG field of byte 2, D = 0
• word operation, W = 1
• BYTE 1 = 00000001
• REG = AX = 000
• Second operand is in memory, MOD = 10
• Effective address, R/M = 001
• BYTE 2 = 10000001
• BYTE 3 = 34H
• BYTE 4 = 12H
• ADD [BX][DI]+1234H, AX is 01813412H (Little Endian)

Example 5

Based-Indexed Addressing Mode

MOV WORD PTR [BP][DI] + 1234H, 0ABCDH
; WORD PTR指標(可寫可不寫)，指向[BP][DI] + 1234H位址
; 0ABCDH立即資料

• Move the immediate data word ABCD16 into the memory location specified by based-indexed address mode
• Byte 1 shown in Fig. 3.6 for the MOV is 1100011W (寬度，1
  $\to$ 2 Bytes)
• word operation, W = 1
• BYTE 1 = 11000111 = C7H
• Byte 2 shown in Fig. 3.6 for MOV is
  $(MOD)000(R/M)$
• Memory operation using 16-bit displacement, MOD = 10, R/M = 011
• BYTE 2 = 10000011
• BYTE 3 = 34H
• BYTE 4 = 12H
• BYTE5 and BYTE6 encode the immediate data = CDAB
• MOV WORD PTR [BP][DI]+1234H, 0ABCDH is C7833412CDABH

詳細指令請看課本第79頁

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REP instruction

repeat

• REP = 1111001Z (Zero Flag)
  • Z is 1or 0 depending on whether the repeat operation is to be done when the zero flag is set or when it is reset

Encoding a complete program in machine cod

參考資料

• Barry B. Bery, “The Intel Microprocessors,” 8th Edition, 2009, Prentice Hall.
• Walter A. Triebel, Avtar Singh, “The 8088 and 8086 Microprocessors – Programming, Interfacing, Software, Hardware, and Applications,” 4th Edition, 2003, Prentice Hall.
• 國立臺北大學資工系張玉山教授ppt