Section 06 Part 06 – Signed Branches (BGE, BGT, BLE, BLT)

“Truth is certainly a branch of morality and a very important one to society.” ~Thomas Jefferson

Introduction

These branches are designed to branch if larger or smaller than a specific value. These deal with signed results, that is to say for using a byte; 80 is the lowest value whilst 7F is the highest.

So when comparing say FE to 02. FE will be considered lower than 02.

The BGE Instruction

BGE – Branch on Greater than or Equal

The destination operand will be added to the PC, and the 68k will continue reading at the new offset held in PC, if the following conditions are met:

1. The N and V flags are both clear
2. The N and V flags are both set

Otherwise, the instruction is ignored.

Examples

This instruction uses the N and V flags together to detect if the result is greater than or equal:

cmpi.w #$0020,d0

bge.s Is20OrHigher

move.w #$0000,d0

Is20OrHigher:

We’ll pretend d0 contains 00009800, the CMP is word, so the comparison is 0020 & 9800.

• 9800 is not higher than or equal to 0020, so the branch is ignored.

Now let’s pretend d0 contains 00000492. The comparison is 0020 & 0492.

• 0492 is higher than 0020, so the 68k will branch to “Is20OrHigher”

The BGT Instruction

BGT – Branch on Greater Than

The destination operand will be added to the PC, and the 68k will continue reading at the new offset held in PC, if the following conditions are met:

1. The Z, N and V flags are all clear
2. The Z flag is clear, but the N and V flags are both set

Otherwise, the instruction is ignored.

Examples

This instruction uses the Z, N and V flags together to detect if the result is greater than:

cmpi.w #$0020,d0

bgt.s IsHigher

move.w #$0000,d0

IsHigher:

We’ll pretend d0 contains 00000020, the CMP is word, so the comparison is 0020 & 0020.

• 0020 is not higher than 0020, so the branch is ignored.

Now let’s pretend d0 contains 00000492. The comparison is 0020 & 0492.

• 0492 is higher than 0020, so the 68k will branch to “IsHigher”

The BLE Instruction

BLE – Branch on Less than or Equal

The destination operand will be added to the PC, and the 68k will continue reading at the new offset held in PC, if the following conditions are met:

1. The Z flag is clear
2. The N flag is clear, but the V flag is set
3. The N flag is set, but the V flag is clear

Otherwise, the instruction is ignored.

Examples

This instruction is the polar opposite of BGE:

cmpi.w #$0020,d0

ble.s Is20OrLower

move.w #$0000,d0

Is20OrLower:

We’ll pretend d0 contains 00009800, the CMP is word, so the comparison is 0020 & 9800.

• 9800 is lower than 0020, so the 68k will branch to “Is20OrLower”

Now let’s pretend d0 contains 00000492. The comparison is 0020 & 0492.

• 0492 is not lower than or equal to 0020, so the branch is ignored.

The BLT Instruction

BLT – Branch on Lower Than

The destination operand will be added to the PC, and the 68k will continue reading at the new offset held in PC, if the following conditions are met:

1. The N flag is clear, but the V flag is set
2. The N flag is set, but the V flag is clear

Otherwise, the instruction is ignored.

Examples

This instruction is the polar opposite of BGT:

cmpi.w #$0020,d0

blt.s IsLower

move.w #$0000,d0

IsLower:

We’ll pretend d0 contains 00000020, the CMP is word, so the comparison is 0020 & 0020.

• 0020 is not lower than 0020, so the branch is ignored.

Now let’s pretend d0 contains 00008492. The comparison is 0020 & 8492.

• 8492 is lower than 0020, so the 68k will branch to “IsLower”

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