user manual

MicroBlaze Processor Reference Guide www.xilinx.com 35
UG081 (v6.0) June 1, 2006 1-800-255-7778
Reset, Interrupts, Exceptions, and Break
R
• Unaligned Exception
The unaligned exception is caused by a word accesswherethe address to the data bus
has bits 30 or 31 set, or a half-word access with bit 31 set.
• Divide by Zero Exception
The divide-by-zeroexception iscauses byan integer division(idiv or idivu)wherethe
divisor is zero.
• FPU Exception
An FPU exception is caused by an underflow, overflow, divide-by-zero, illegal
operation, or denormalized operand occurring with a floating point instruction.
♦ Underflow occurs when the result is denormalized.
♦ Overflow occurs when the result is not-a-number (NaN).
♦ The divide-by-zero FPU exception is caused by the rA operand to fdiv being zero
when rB is not infinite.
♦ Illegal operation is caused by a signaling NaN operand or by illegal infinite or
zero operand combinations.
Equivalent Pseudocode
r17 ← PC
PC
← 0x00000020
MSR[EE]
← 0
MSR[EIP]
← 1
ESR[DS]
← exception in delay slot
ESR[EC]
← exception specific value
ESR[ESS]
← exception specific value
EAR
← exception specific value
FSR
← exception specific value
Breaks
There are two kinds of breaks:
• Hardware (external) breaks
• Software (internal) breaks
Hardware Breaks
Hardware breaks are performed by asserting the external break signal (i.e. the Ext_BRK
and Ext_NM_BRK input ports). On a break the instruction in the execution stage will
complete, while the instruction in the decode stage is replaced by a branch to the break
vector (address 0x18). The break return address (the PC associated with the instruction in
the decode stage at the time of the break) is automatically loaded into general purpose
register R16. MicroBlaze also sets the Break In Progress (BIP) flag in the Machine Status
Register (MSR).
A normal hardware break (i.e the Ext_BRK input port) is only handled when there is no
break in progress (i.e MSR[BIP] is set to 0). The Break In Progress flag disables interrupts.
A non-maskable break (i.e the Ext_NM_BRK input port) will always be handled
immediately.
The BIP bit in the MSR is automatically cleared when executing the RTBD instruction.