C8051F54X_14 Datasheet, PDF (87/275 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F54x
11.2.1.1. General Purpose Registers
The lower 32 bytes of data memory, locations 0x00 through 0x1F, may be addressed as four banks of gen-
eral-purpose registers. Each bank consists of eight byte-wide registers designated R0 through R7. Only
one of these banks may be enabled at a time. Two bits in the program status word, RS0 (PSW.3) and RS1
(PSW.4), select the active register bank (see description of the PSW in SFR Definition 10.6). This allows
fast context switching when entering subroutines and interrupt service routines. Indirect addressing modes
use registers R0 and R1 as index registers.
11.2.1.2. Bit Addressable Locations
In addition to direct access to data memory organized as bytes, the sixteen data memory locations at 0x20
through 0x2F are also accessible as 128 individually addressable bits. Each bit has a bit address from
0x00 to 0x7F. Bit 0 of the byte at 0x20 has bit address 0x00 while bit7 of the byte at 0x20 has bit address
0x07. Bit 7 of the byte at 0x2F has bit address 0x7F. A bit access is distinguished from a full byte access by
the type of instruction used (bit source or destination operands as opposed to a byte source or destina-
tion).
The MCS-51™ assembly language allows an alternate notation for bit addressing of the form XX.B where
XX is the byte address and B is the bit position within the byte. For example, the instruction:
MOV C, 22.3h
moves the Boolean value at 0x13 (bit 3 of the byte at location 0x22) into the Carry flag.
11.2.1.3. Stack
A programmer's stack can be located anywhere in the 256-byte data memory. The stack area is desig-
nated using the Stack Pointer (SP) SFR. The SP will point to the last location used. The next value pushed
on the stack is placed at SP+1 and then SP is incremented. A reset initializes the stack pointer to location
0x07. Therefore, the first value pushed on the stack is placed at location 0x08, which is also the first regis-
ter (R0) of register bank 1. Thus, if more than one register bank is to be used, the SP should be initialized
to a location in the data memory not being used for data storage. The stack depth can extend up to
256 bytes.
11.3. External RAM
For C8051F54x devices, 1 kB of RAM are included on-chip and mapped into the external data memory
space (XRAM). The external memory space may be accessed using the external move instruction (MOVX)
and the data pointer (DPTR), or using the MOVX indirect addressing mode using R0 or R1. If the MOVX
instruction is used with an 8-bit address operand (such as @R1), then the high byte of the 16-bit address is
provided by the External Memory Interface Control Register (EMI0CN, shown in SFR Definition 11.1).
Note: The MOVX instruction can also be used for writing to the Flash memory. See Section “14. Flash Memory” on
page 117 for details. The MOVX instruction accesses XRAM by default.
11.3.1. 16-Bit MOVX Example
The 16-bit form of the MOVX instruction accesses the memory location pointed to by the contents of the
DPTR register. The following series of instructions reads the value of the byte at address 0x1234 into the
accumulator A:
MOV DPTR, #1234h
; load DPTR with 16-bit address to read (0x1234)
MOVX A, @DPTR
; load contents of 0x1234 into accumulator A

The above example uses the 16-bit immediate MOV instruction to set the contents of DPTR. Alternately,
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