DSPIC30F3011-20E Datasheet, PDF (18/226 Pages) Microchip Technology – High-Performance, 16-Bit Digital Signal Controllers

dsPIC30F3010/3011
2.2 Programmer’s Model
The programmer’s model is shown in Figure 2-1 and
consists of 16x16-bit working registers (W0 through
W15), 2x40-bit accumulators (ACCA and ACCB),
STATUS Register (SR), Data Table Page register
(TBLPAG), Program Space Visibility Page register
(PSVPAG), DO and REPEAT registers (DOSTART,
DOEND, DCOUNT and RCOUNT) and Program Coun-
ter (PC). The working registers can act as Data,
Address or Offset registers. All registers are memory
mapped. W0 acts as the W register for file register
addressing.
Some of these registers have a Shadow register asso-
ciated with each of them, as shown in Figure 2-1. The
Shadow register is used as a temporary holding regis-
ter and can transfer its contents to or from its host reg-
ister upon the occurrence of an event. None of the
Shadow registers are accessible directly. The following
rules apply for transfer of registers into and out of
shadows.
• PUSH.S and POP.S
W0, W1, W2, W3, SR (DC, N, OV, Z and C bits
only) are transferred.
• DO instruction
DOSTART, DOEND, DCOUNT shadows are
pushed on loop start, and popped on loop end.
When a byte operation is performed on a working reg-
ister, only the Least Significant Byte (LSB) of the target
register is affected. However, a benefit of memory
mapped working registers is that both the Least and
Most Significant Bytes can be manipulated through
byte-wide data memory space accesses.
2.2.1
SOFTWARE STACK POINTER/
FRAME POINTER
The dsPIC^® DSC devices contain a software stack.
W15 is the dedicated Software Stack Pointer, and will
be automatically modified by exception processing and
subroutine calls and returns. However, W15 can be ref-
erenced by any instruction in the same manner as all
other W registers. This simplifies the reading, writing
and manipulation of the Stack Pointer (e.g., creating
stack frames).
Note: In order to protect against misaligned
stack accesses, W15<0> is always clear.
W15 is initialized to 0x0800 during a Reset. The user
may reprogram the SP during initialization to any
location within data space.
W14 has been dedicated as a Stack Frame Pointer as
defined by the LNK and ULNK instructions. However,
W14 can be referenced by any instruction in the same
manner as all other W registers.
2.2.2 STATUS REGISTER
The dsPIC DSC core has a 16-bit STATUS Register
(SR), the LSB of which is referred to as the SR Low
Byte (SRL) and the MSB as the SR High Byte (SRH).
See Figure 2-1 for SR layout.
SRL contains all the MCU ALU operation status flags
(including the Z bit), as well as the CPU Interrupt Prior-
ity Level status bits, IPL<2:0>, and the Repeat Active
status bit, RA. During exception processing, SRL is
concatenated with the MSB of the PC to form a
complete word value which is then stacked.
The upper byte of the SR register contains the DSP
adder/subtracter status bits, the DO Loop Active bit
(DA) and the Digit Carry (DC) status bit.
2.2.3 PROGRAM COUNTER
The Program Counter is 23 bits wide. Bit 0 is always
clear. Therefore, the PC can address up to 4M
instruction words.
DS70141E-page 16
© 2008 Microchip Technology Inc.