SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 19

• Manufacturer Part Number: SAB80C517-M16
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

4.3
The lower 32 locations of the internal RAM are assigned to four banks with eight general purpose
registers (GPRs) each. Only one of these banks may be enabled at a time. Two bits in the program
status word, PSW.3 and PSW.4, select the active register bank (see description of the PSW). This
allows fast context switching, which is useful when entering subroutines or interrupt service
routines. ASM51 and the device SAB 80C517 default to register bank 0.
The 8 general purpose registers of the selected register bank may be accessed by register
addressing. With register addressing the instruction of code indicates which register is to be used.
For indirect addressing R0 and R1 are used as pointer or index register to address internal or
external memory (e.g. MOV @R0).
Reset initializes the stack pointer to location 07 H and increments it once to start from location 08 H
which is also the first register (R0) of register bank 1. Thus, if one is going to use more than one
register bank, the SP should be initialized to a different location of the RAM which is not used for
data storage.
4.4
The special function register (SFR) area has two important functions. Firstly, all CPU registers
except the program counter and the four register banks reside here. The CPU registers are the
arithmetic registers like A, B, PSW and pointers like SP, DPHx and DPLx.
Secondly, a number of registers constitute the interface between the CPU and all on-chip
peripherals. That means, all control and data transfers from and to the peripherals use this register
interface exclusively.
The special function register area is located in the address space above the internal RAM from
addresses 80 H to FF H . All 81 special function registers of the SAB 80C517 reside here.
Sixteen SFRs, that are located on addresses dividable by eight, are bit-addressable, thus allowing
128 bit-addressable locations within the SFR area.
Since the SFR area is memory mapped, access to the special function registers is as easy as with
the internal RAM, and they may be processed with most instructions. In addition, if the special
functions are not used, some of them may be used as general scratch pad registers. Note, however,
all SFRs can be accessed by direct addressing only.
The special function registers are listed in the following tables where they are organized in
functional groups which refer to the functional blocks of the SAB 80C517. Block names and symbols
are listed in alphabetical order. Bit addressable special function registers are marked with a dot in
the fifth column. Special function registers with bits belonging to more then one functional block are
marked with an asterisk at the symbol name.
Semiconductor Group
General Purpose Registers
Special Function Registers
20
Memory Organization