Datasheet

dsPIC30F4011/4012
DS70135G-page 18 © 2010 Microchip Technology Inc.
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 register
and can transfer its contents to or from its host register
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
register, only the Least Significant Byte 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 Digital Signal Controllers contain a software
stack. W15 is the dedicated software Stack Pointer
(SP) and is automatically modified by exception pro-
cessing and subroutine calls and returns. However,
W15 can be referenced 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).
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 Least Significant Byte of which is referred to as the
SR Low Byte (SRL) and the Most Significant Byte as the
SR High Byte (SRH). See Figure 2-1 for SR layout.
SRL contains all the DSP 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.
Note: In order to protect against misaligned
stack accesses, W15<0> is always clear.