An apparatus comprising: processing circuitry to perform data processing in response to instructions, the processing circuitry comprising transactional memory support circuitry to support execution of a transaction within a thread of data processing by the processing circuitry, the transaction comprising instructions of the thread executed speculatively between a transaction start instruction and a transaction end instruction, for which the processing circuitry is configured to prevent commitment of results of the speculatively executed instructions until the transaction end instruction is reached, and to abort processing of the transaction when an abort event occurs before the transaction end instruction is reached; and exception handling circuitry to determine whether to mask an exception in dependence on exception mask information specifying whether one or more subsets of exceptions are enabled or disabled; wherein in response to an exception mask update instruction executed within a transaction to update the exception mask information to enable at least one subset of exceptions which was disabled at the start of processing of the transaction, the processing circuitry is configured to permit unaborted processing of one or more subsequent instructions of the transaction following said exception mask update instruction, with said exception mask update instruction not causing an abort event unless another cause of an abort event is also detected; wherein the processing circuitry is configured to trigger said abort event in response to occurrence of an enabled exception during processing of a transaction; wherein in response to occurrence of an enabled exception during processing of a transaction, when said enabled exception was disabled at the start of processing of the transaction, the processing circuitry is configured to set abort status information to a predetermined value; wherein in response to an abort event occurring for a transaction, when the abort event is triggered by an event other than occurrence of an enabled exception or is triggered by occurrence of an enabled exception which was enabled at the start of processing of the transaction, the processing circuitry is configured to set said abort status information to a value other than said predetermined value. processing circuitry to perform data processing in response to instructions, the processing circuitry comprising transactional memory support circuitry to support execution of a transaction within a thread of data processing by the processing circuitry, the transaction comprising instructions of the thread executed speculatively between a transaction start instruction and a transaction end instruction, for which the processing circuitry is configured to prevent commitment of results of the speculatively executed instructions until the transaction end instruction is reached, and to abort processing of the transaction when an abort event occurs before the transaction end instruction is reached; and exception handling circuitry to determine whether to mask an exception in dependence on exception mask information specifying whether one or more subsets of exceptions are enabled or disabled; wherein in response to an exception mask update instruction executed within a transaction to update the exception mask information to enable at least one subset of exceptions which was disabled at the start of processing of the transaction, the processing circuitry is configured to permit unaborted processing of one or more subsequent instructions of the transaction following said exception mask update instruction, with said exception mask update instruction not causing an abort event unless another cause of an abort event is also detected; wherein the processing circuitry is configured to trigger said abort event in response to occurrence of an enabled exception during processing of a transaction; wherein in response to occurrence of an enabled exception during processing of a transaction, when said enabled exception was disabled at the start of processing of the transaction, the processing circuitry is configured to set abort status information to a predetermined value; wherein in response to an abort event occurring for a transaction, when the abort event is triggered by an event other than occurrence of an enabled exception or is triggered by occurrence of an enabled exception which was enabled at the start of processing of the transaction, the processing circuitry is configured to set said abort status information to a value other than said predetermined value.