National CP3BT26 - Manual (Page 19)

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CP3
BT26
5.6 STACKS
A stack is a last-in, first-out data structure for dynamic stor-
age of data and addresses. A stack consists of a block of
memory used to hold the data and a pointer to the top of the
stack. As more data is pushed onto a stack, the stack grows
downward in memory. The CR16C supports two types of
stacks: the interrupt stack and program stacks.
5.6.1 Interrupt Stack
The processor uses the interrupt stack to save and restore
the program state during the exception handling. Hardware
automatically pushes this data onto the interrupt stack be-
fore entering an exception handler. When the exception
handler returns, hardware restores the processor state with
data popped from the interrupt stack. The interrupt stack
pointer is held in the ISP register.
5.6.2 Program Stack
The program stack is normally used by software to save and
restore register values on subroutine entry and exit, hold lo-
cal and temporary variables, and hold parameters passed
between the calling routine and the subroutine. The only
hardware mechanisms which operate on the program stack
are the PUSH, POP, and POPRET instructions.
5.6.3 User and Supervisor Stack Pointers
To support multitasking operating systems, support is pro-
vided for two program stack pointers: a user stack pointer
and a supervisor stack pointer. When the PSR.U bit is clear,
the SP register is used for all program stack operations. This
is the default mode when the user/supervisor protection
mechanism is not used, and it is the supervisor mode when
protection is used.
When the PSR.U bit is set, the processor is in user mode,
and the USP register is used as the program stack pointer.
User mode can only be entered using the JUSR instruction,
which performs a jump and sets the PSR.U bit. User mode
is exited when an exception is taken and re-entered when
the exception handler returns. In user mode, the LPRD in-
struction cannot be used to change the state of processor
registers (such as the PSR).
5.7 INSTRUCTION SET
Table 4 lists the operand specifiers for the instruction set,
and Table 5 is a summary of all instructions. For each in-
struction, the table shows the mnemonic and a brief de-
scription of the operation performed.
In the mnemonic column, the lower-case letter “i” is used to
indicate the type of integer that the instruction operates on,
either “B” for byte or “W” for word. For example, the notation
ADDi for the “add” instruction means that there are two
forms of this instruction, ADDB and ADDW, which operate
on bytes and words, respectively.
Similarly, the lower-case string “cond” is used to indicate the
type of condition tested by the instruction. For example, the
notation Jcond represents a class of conditional jump in-
structions: JEQ for Jump on Equal, JNE for Jump on Not
Equal, etc. For detailed information on all instructions, see
the CompactRISC CR16C Programmer's Reference Manu-
al.
Table 4 Key to Operand Specifiers
Operand Specifier Description
abs Absolute address
disp
Displacement (numeric suffix
indicates number of bits)
imm
Immediate operand (numeric suf-
fix indicates number of bits)
Iposition Bit position in memory
Rbase Base register (relative mode)
Rdest Destination register
Rindex Index register
RPbase, RPbasex Base register pair (relative mode)
RPdest Destination register pair
RPlink Link register pair
Rposition Bit position in register
Rproc 16-bit processor register
Rprocd 32-bit processor register
RPsrc Source register pair
RPtarget Target register pair
Rsrc, Rsrc1, Rsrc2 Source register