ST6391 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ST6391' PDF : 68 Pages View PDF
ST6391,92,93,95,97,99
ST639x CORE (Continued)
Indirect Registers (X, Y). These two indirect reg-
isters are used as pointers to the memory locations
in the data space. They are used in the register-in-
direct addressing mode.These registers can be
addressed in the data space as RAM locations at
the 80h (X) and 81h (Y) addresses. They can also
be accessed with the direct, short direct, or bit di-
rect addressing modes. Accordingly, the ST639x
instruction set can use the indirect registers as any
other register of the data space.
Short Direct Registers (V, W). These two regis-
ters are used to save one byte in short direct ad-
dressing mode. These registers can be addressed
in the data space as RAM locations at the 82h (V)
and 83h (W) addresses. They can also be ac-
cessed with the direct and bit direct addressing
modes. Accordingly, the ST639x instruction set
can use the short direct registers as any other reg-
ister of the data space.
Program Counter (PC)
The program counteris a 12-bit register thatcontains
the address of the next ROM location to be proc-
essed by the core. This ROM location may be an op-
code, an operand, or an address of operand. The
12-bit length allows the direct addressing of 4096
bytes in the program space. Nevertheless, if the pro-
gram space contains more than 4096 locations, the
further program space can be addressed by using
the Program ROM Page Register. The PC value is
incremented, after it is read for the address of the
current instruction, by sending it through the ALU, so
giving the address of the next byte in the program.
To execute relative jumps the PC and the offset val-
ues are shifted through the ALU, where they will be
added, and the result is shifted back into the PC. The
program counter can be changed in the following
ways:
JP (Jump) instruction.... PC= Jump address
CALL instruction ........... PC=Call address
Relative Branch
instructions ................... PC=PC+offset
Interrupt........................ PC=Interrupt vector
Reset............................ PC=Reset vector
RET & RETI instructions............PC=Pop (stack)
Normal instruction ........ PC= PC+1
Flags (C, Z)
The ST639x Core includes three pairs of flags that
correspond to 3 different modes: normal mode, in-
terrupt mode and Non-Maskable-Interrupt-Mode.
Each pair consists of a CARRY flag and a ZERO
flag. One pair (CN, ZN) is used during normal op-
eration, one pair is used during the interrupt mode
(CI,ZI) and one is used during the not-maskable in-
terrupt mode (CNMI, ZNMI).
The ST639x Core uses the pair of flags that corre-
sponds to the actual mode: as soon as an interrupt
(resp. a Non-Maskable-Interrupt) is generated, the
ST639x Core uses the interrupt flags (resp. the NMI
flags) instead of the normal flags. When the RETI in-
struction is executed, the normal flags (resp. the inter-
rupt flags) are restored if the MCU was in the normal
mode(resp. in the interrupt mode)before the interrupt.
Should be observed that each flag set can only be ad-
dressed in its own routine (Not-maskable interrupt,
normal interrupt or main routine). The interrupt flags
are not cleared during the context switching and so,
theyremain in the state they were at the exit of the last
routine switching.
The Carry flag is set when a carry or a borrow oc-
curs during arithmetic operations, otherwise it is
cleared. The Carry flag is also set to the value of
the bit tested in a bit test instruction, and partici-
pates in the rotate left instruction.
The Zero flag is set if the result of the last arithmetic
or logical operation was equal to zero, otherwise it
is cleared.
The switching between these three sets is auto-
matically performed when an NMI, an interrupt and
a RETI instructions occur. As the NMI mode is
automatically selected after the reset of the MCU,
the ST639x Core uses at first the NMI flags.
Figure 7. Stack Operation
WHEN
RET OR RETI
OCCURS
PROGRAM COUNTER
STACK LEVEL 1
STACK LEVEL 2
STACK LEVEL 3
STACK LEVEL 4
STACK LEVEL 5
STACK LEVEL 6
WHEN CALL
OR
INTERRUPT REQUEST
OCCURS
VA000424
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