Breakdown the jump instruction 0xfe1ff0ef ?

Steps

1. Convert to binary representation
2. Extract the imm fields
3. Reorganize and concatenate
4. Perform 2s complement conversion (if -ve need additional steps like example here, if positive just read out number in binary).

Instruction in Binary (Bits 31 to 0)

First, convert each hexadecimal digit to its 4-bit binary equivalent and write them in order:

Hexadecimal to Binary Conversion:

Hex	Binary
f	1111
e	1110
1	0001
f	1111
f	1111
0	0000
e	1110
f	1111

Combined Binary Instruction:

#### **Binary Representation**

Bit Position	[31]	[30]	[29]	[28]	[27]	[26]	[25]	[24]	[23]	[22]	[21]	[20]	[19]	[18]	[17]	[16]	[15]	[14]	[13]	[12]	[11]	[10]	[9]	[8]	[7]	[6]	[5]	[4]	[3]	[2]	[1]	[0]
Binary Value	1	1	1	1	1	1	1	0	0	0	0	1	1	1	1	1	1	1	1	1	0	0	0	0	1	1	1	0	1	1	1	1

Grouped by Fields

Field	Bits	Binary Value
imm[20]	[31]	1
imm[10:1]	[30‒21]	1 1 1 1 1 1 1 0 0 0
imm[11]	[20]	1
imm[19:12]	[19‒12]	1 1 1 1 1 1 1 1
rd	[11‒7]	0 0 0 0 1
opcode	[6‒0]	1 1 0 1 1 1 1

Field Breakdown

The RISC-V JAL instruction format divides the 32-bit instruction into the following fields:

• imm[20]: Bit [31]
• imm[10:1]: Bits [30:21]
• imm[11]: Bit [20]
• imm[19:12]: Bits [19:12]
• rd: Bits [11:7]
• opcode: Bits [6:0]

Detailed Breakdown:

• Opcode (bits [6:0]): 1 1 0 1 1 1 1 (binary) = 0x6F (hexadecimal)

  • Instruction: JAL (Jump and Link)

• Destination Register (rd) (bits [11:7]): 0 0 0 0 1 (binary) = 1 (decimal)

  • Destination Register: x1

• Immediate Fields:

  • imm[20] (bit [31]): 1
  • imm[10:1] (bits [30:21]): 1 1 1 1 1 1 1 0 0 0
  • imm[11] (bit [20]): 1
  • imm[19:12] (bits [19:12]): 1 1 1 1 1 1 1 1

Immediate Value Reconstruction

To reconstruct the immediate value, we rearrange the bits according to the JAL instruction format:

Immediate[20]    Immediate[10:1]     Immediate[11]    Immediate[19:12]
     1        | 1 1 1 1 1 1 1 0 0 0 |      1        | 1 1 1 1 1 1 1 1

Combined Immediate Bits (before shifting):

Immediate[20:1]: 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0

Note: The immediate value is left-shifted by 1 (since the address is word-aligned), so we append a 0 at the end.

Final Immediate Value (21 bits):

Immediate[20:0]: 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0

• Since the MSB is a 1, the immediate value is negative. It represents -offset.
• Thus, we first figure out what +offset is in decimal and then apply the negative sign.
• To find out what +offset is we apply the following steps in binary form. -(-offset) = ~offset + 1

Calculating the Offset (Two's Complement)

Since the most significant bit is 1, the immediate value is negative.

• Two's Complement Calculation:

  1. Invert the Bits:

     Inverted: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1
     

  2. Add 1:

     Two's Complement Value: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0
     

• Convert to Decimal:

  Binary: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0
  Decimal Value: 32
  

• Apply Negative Sign:

  Offset = -32 bytes
  

Final Disassembled Instruction

jal x1, -32

Breakdown the branch instruction 0xfecd80e3

This detailed breakdown shows the step-by-step disassembly of the branch instruction , including the binary conversion, field extraction, immediate value calculation, and the final assembly instruction.

Instruction Breakdown Table

Bit Position	[31]	[30]	[29]	[28]	[27]	[26]	[25]	[24]	[23]	[22]	[21]	[20]	[19]	[18]	[17]	[16]	[15]	[14]	[13]	[12]	[11]	[10]	[9]	[8]	[7]	[6]	[5]	[4]	[3]	[2]	[1]	[0]
Binary Value	1	1	1	1	1	1	1	0	1	1	0	0	1	1	0	1	1	0	0	0	0	0	0	0	1	1	1	0	0	0	1	1

Field Breakdown Table

Field	Bits	Binary Value	Description
opcode	[6:0]	1 1 0 0 0 1 1	Opcode (0x63)
funct3	[14:12]	0 0 0	Function (BEQ)
rs1	[19:15]	1 1 0 1 1	Source Register 1 (x27)
rs2	[24:20]	0 1 1 0 0	Source Register 2 (x12)
imm[12]	[31]	1	Immediate bit 12
imm[10:5]	[30:25]	1 1 1 1 1 1	Immediate bits 10:5
imm[4:1]	[11:8]	0 0 0 0	Immediate bits 4:1
imm[11]	[7]	1	Immediate bit 11

Immediate Value Reconstruction

Immediate Bits in Order:

Immediate Bit	Value	Source Bit Position
[12]	1	[31]
[11]	1	[7]
[10]	1	[30]
[9]	1	[29]
[8]	1	[28]
[7]	1	[27]
[6]	1	[26]
[5]	1	[25]
[4]	0	[11]
[3]	0	[10]
[2]	0	[9]
[1]	0	[8]
[0]	0	(After shifting left)

Immediate Binary Value (After Shifting Left by 1):

Immediate[12:0]: 1 1 1 1 1 1 1 1 0 0 0 0 0

Calculating Offset:

• Two's Complement:

  Inverted: 0 0 0 0 0 0 0 0 1 1 1 1 1
  Add 1:    +                             1
             -----------------------------
             0 0 0 0 0 0 0 1 0 0 0 0 0
  

• Decimal Value: 32

• Offset: -32 bytes

Final Disassembled Instruction

beq x27, x12, -32

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