NEC Network Controller uPD98502 - Manual (Page 136)

CHAPTER 2 V R 4120A
136 Preliminary User’s Manual S15543EJ1V0UM
(7) Status after reset
The contents of the Status register are undefined after Cold resets, except for the following bits in the diagnostic
status field.
• TS and SR are cleared to 0.
• ERL and BEV are set to 1.
• SR is 0 after Cold reset, and is 1 after Soft reset or NMI interrupt.
Remark Cold reset and Soft reset are CPU core reset (see Section 2.6 Initialization Interface).
2.5.3.6 Cause register (13)
The 32-bit read/write Cause register holds the cause of the most recent exception. A 5-bit exception code
indicates one of the causes (see Table 2-35). Other bits hold the detailed information of the specific exception. All
bits in the Cause register, with the exception of the IP1 and IP0 bits, are read-only; IP1 and IP0 are used for software
interrupts. Figure 2-53 shows the fields of this register; Table 2-35 describes the Cause register codes.
Figure 2-53. Cause Register Format
8 27 16 15 6 7 2 1 0
12 8 1 5 2
31 30 29 28
BD 0 CE
2 1 1
0 IP(7:0) 0 ExcCode 0
BD : Indicates whether the most recent exception occurred in the branch delay slot (1
→ In delay slot, 0
→ Normal).
CE : Indicates the coprocessor number in which a Coprocessor Unusable exception occurred.
This field will remain undefined for as long as no exception occurs.
IP : Indicates whether an interrupt is pending (1
→ Interrupt pending, 0 → No interrupt pending).
IP7 : A timer interrupt.
IP(6:2) : Ordinary interrupts (Int(4:0)
Note
). However, Int4
Note
never occurs in the V R 4120A CPU.
IP(1:0) : Software interrupts. Only these bits cause an interrupt exception, when they are set to 1 by
means of software.
Note Int (4:0) are internal signals of the CPU core. For details about connection to the on-chip
peripheral units.
ExcCode: Exception code field (refer to Table 2-35 for details).
0
: RFU. Write 0 in a write operation. When this field is read, 0 is read.