2016-06-18 - In IL: Branching Instructions

In IL

• Part 1 - Introduction
• Part 2 - Variables and Types
• Part 3 - Variables in Visual Basic .NET
• Part 4 - Instructions and the Stack
• Part 5 - Volume of a Cylinder (Operations)
• Part 6 - Branching Instructions
• Part 7 - Largest of Two Numbers (if-else)
• Part 8 - Largest of Three Numbers (If-ElseIf-Else)
• Part 9 - Switch instruction
• Part 10 - Grade Analyser (switch)
• Part 11 - Prize Calculator (switch-2)
• Part 12 - VB Grade Analyser (Select)
• Part 13 - Loop Instructions
• Part 14 - Print the Alphabet (while)
• Part 15 - Print the Alphabet (do while, for)
• Part 16 - Print the Alphabet (Do Until)
• Part 17 - Print the Alphabet (break, continue)
• Part 18 - Array Instructions
• Part 19 - Summing Arrays
• Part 20 - Other Instructions
• Part 21 - Assemblies
• Part 22 - Class Definitions
• Part 23 - C# Classes and Structs
• Part 24 - VB Classes, Modules and Structures
• Part 25 - Field Definitions
• Part 26 - Field Declarations

Conditional logic is what allows a program to appear as though it's making decisions. Certain parts of code are executed only if specific conditions are met. In higher level programming languages this would be handled by if, switch, or select statements. In IL these decisions are handled by branching instructions. Instructions that change the next instruction to be executed if certain conditions are true.

Generally instructions are executed one after the other. A branching instruction causes a jump. Instead of executing the next instruction it jumps to the instruction indicated by the branching instruction. It then continues to executes instructions after that point until another branching instruction is found.

br (BRanch)

The unconditional branch instruction always jumps to the instruction indicated by its argument.

brfalse (BRanch FALSE), brtrue (BRanch TRUE)

Pops a single value off of the stack. The brfalse instruction jumps to the instruction indicated by its argument if the value is 0. The brtrue instruction jumps to the instruction indicated by its argument if the value is nonzero.

The brfalse instruction is aliased as brnull (BRanch NULL) and brzero (BRanch ZERO). The brtrue function is aliased as brinst (BRanch INSTance)

beq (Branch EQual), ceq (Compare EQual)

Pops two values off of the stack, and compares them. The branching instruction jumps to the instruction indicated by its argument if they are equal. The compare instruction pushes 1 onto the stack if they are equal and 0 otherwise.

bge (Branch Greater-than or Equal)

Pops two values off of the stack, and compares them. The branching instruction jumps to the instruction indicated by its argument if the second value popped is greater than or equal to the first.

bgt (Branch Greater-Than), cgt (Compare Greater-Than)

Pops two values off of the stack, and compares them. The branching instruction jumps to the instruction indicated by its argument if the second value popped is greater than the first. The compare instruction pushes 1 onto the stack if the second value popped is greater than the first and 0 otherwise.

ble (Branch Less-than or Equal)

Pops two values off of the stack, and compares them. The branching instruction jumps to the instruction indicated by its argument if the second value popped is less than or equal to the first.

blt (Branch Less-Than), clt (Compare Less-Than)

Pops two values off of the stack, and compares them. The branching instruction jumps to the instruction indicated by its argument if the second value popped is less than the first. The compare instruction pushes 1 onto the stack if the second value popped is less than the first and 0 otherwise.

bne (Branch Not Equal)

Pops two values off of the stack, and compares them. The branching instruction jumps to the instruction indicated by its argument if they are not equal.

and, or, xor, not

These instructions perform the specified bitwise action on values popped from the stack. The not instruction takes a single value while the rest take two.

Next time we will look at if statements and how they get represented in IL.