Toshiba TLCS-900/L1 Series Manual page 41

3.4.1 General-purpose Interrupt Processing
When the CPU accepts an interrupt, it usually performs the following sequence of
operations. That is also the same as TLCS-900/L and TLCS-900/H.
(1) The CPU reads the interrupt vector from the interrupt controller.
If the same level interrupts occur simultaneously, the interrupt controller generates
an interrupt vector in accordance with the default priority and clears the interrupt
request.
(The default priority is already fixed for each interrupt: The smaller vector value has
the higher priority level.)
(2) The CPU pushes the value of program counter (PC) and status register (SR) onto the
stack area (Indicated by XSP).
(3) The CPU sets the value which is the priority level of the accepted interrupt plus 1
(+1) to the interrupt mask register <IFF2:0>. However, if the priority level of the
accepted interrupt is 7, the register's value is set to 7.
(4) The CPU increases the interrupt nesting counter INTNEST by 1 (+1).
(5) The CPU jumps to the address indicated by the data at address FFFF00H + interrupt
vector and starts the interrupt processing routine.
(6) The above processing time is 18-states (1.09 µs at 33 MHz) as the best case (16 bits
data bus width and 0 waits).
When the CPU compled the interrupt processing, use the RETI instruction to
return to the main routine. RETI restores the contents of program counter (PC) and
status register (SR) from the stack and decreases the interrupt nesting counter
INTNEST by 1 (−1).
Non-maskable interrupts cannot be disabled by a user program. Maskable
interrupts, however, can be enabled or disabled by a user program. A program can set
the priority level for each interrupt source. (A priority level setting of 0 or 7 will
disable an interrupt request.)
If an interrupt request which has a priority level equal to or greater than the value
of the CPU interrupt mask register <IFF2:0> comes out, the CPU accepts its
interrupt. Then, the CPU interrupt mask register <IFF2:0> is set to the value of the
priority level for the accepted interrupt plus 1 (+1).
Therefore, if an interrupt is generated with a higher level than the current
interrupt during its processing, the CPU accepts the later interrupt and goes to the
nesting status of interrupt processing.
Moreover, if the CPU receives another interrupt request while performing the said
(1) to (5) processing steps of the current interrupt, the latest interrupt request is
sampled immediately after execution of the first instruction of the current interrupt
processing routine. Specifying DI as the start instruction disables maskable interrupt
nesting.
A reset initializes the interrupt mask register <IFF2:0> to 111, disabling all
maskable interrupts.
Table 3.4.1 shows the TMP91C824 interrupt vectors and micro DMA start vectors.
The address FFFF00H to FFFFFFH (256 bytes) is assigned for the interrupt vector
area.
91C824-39
TMP91C824
2008-02-20