MITSUBISHI MICROCOMPUTERS
M37754M8C-XXXGP, M37754M8C-XXXHP
M37754S4CGP, M37754S4CHP
SINGLE-CHIP 16-BIT CMOS MICROCOMPUTER
5. Index register length flag (x)
9. Processor interrupt priority level (IPL)
The index register length flag determines whether index register X
and index register Y are used as 16-bit registers or as 8-bit registers.
The registers are used as 16-bit registers when flag x is “0” and as 8-
bit registers when it is “1”.
The processor interrupt priority level (IPL) consists of 3 bits and de-
termines the priority of processor interrupts from level 0 to level 7.
Interrupt is enabled when the interrupt priority of the device request-
ing interrupt (set using the interrupt control register) is higher than the
processor interrupt priority. When interrupt is enabled, the current
processor interrupt priority level is saved in a stack and the proces-
sor interrupt priority level is replaced by the interrupt priority level of
the device requesting the interrupt. Refer to the section on interrupts
for more details.
This flag can be set and reset with the SEP and CLP instructions.
6. Data length flag (m)
The data length flag determines whether the data length is 16-bit or
8-bit. The data length is 16-bit when flag m is “0” and 8-bit when it is
“1”. This flag can be set and reset with the SEM and CLM instructions
or with the SEP and CLP instructions.
Note: Fix bits 11 to 15 of the processor status register (PS) to “0”.
BUS INTERFACE UNIT
The CPU operates on the basis of internal clock φ CPU frequency. In
order to speed-up processing, a bus interface unit is used to pre-
fetch instructions when the data bus is idle. The bus interface unit
synchronizes the CPU and the bus and pre-fetches instructions. Fig-
ure 4 shows the relationship between the CPU and the bus interface
unit.
7. Overflow flag (V)
The overflow flag is valid when addition or subtraction is performed
with a word treated as a signed binary number. If data length flag m
is “0”, the overflow flag is set when the result of addition or subtrac-
tion is outside the range between –32768 and +32767. If data length
flag m is “1”, the overflow flag is set when the result of addition or
subtraction is outside the range between –128 and +127. It is reset
in all other cases. The overflow flag can also be set and reset directly
with the SEP, and CLV or CLP instructions.
The bus interface unit controls buses to access memories easily.
Refer to BUS CYCLE on the following pages. The bus interface unit
has a program address register, a 3-byte instruction queue buffer, a
data address register, and a 2-byte data buffer.
Additionally, the overflow flag is set when a result of unsigned/signed
division exceeds the length of the register where the result is to be
stored; the flag is also set when the addition result is outside range
of –2147483648 to +2147483647 in the RMPA operation.
The bus interface unit obtains an instruction code from memory and
stores it in the instruction queue buffer, obtains data from memory
and stores it in the data buffer, or writes the data form the data buffer
to the memory.
8. Negative flag (N)
The negative flag is set when the result of arithmetic operation or
data transfer is negative (If data length flag m is “0”, data’s bit 15 is
“1”. If data length flag m is “1”, data’s bit 7 is “1”.) It is reset in all other
cases. It can also be set and reset with the SEP and CLP instruc-
tions.
D'8–D'15
D'0–D'7
A'0–A'23
D8–D15
D0–D7
A0–A23
BHE
Bus interface
CPU
WR
unit
RD
ALE
Control signal
BYTE
HOLD
Fig. 4 Relationship between the CPU and the bus interface unit
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RENESAS [ RENESAS TECHNOLOGY CORP ]
