ST92F124/F150/F250 - DEVICE ARCHITECTURE
MMU REGISTERS (Cont’d)
2.7.2 CSR: Code Segment Register
ISR and ENCSR bit (EMR2 register) are also de-
scribed in the chapter relating to Interrupts, please
refer to this description for further details.
This register selects the 64-Kbyte code segment
being used at run-time to access instructions. It
can also be used to access data if the spminstruc-
tion has been executed (or ldpp, ldpd, lddp).
Only the 6 LSBs of the CSR register are imple-
mented, and bits 6 and 7 are reserved. The CSR
register allows access to the entire memory space,
divided into 64 segments of 64 Kbytes.
Bits 7:6 = Reserved, keep in reset state.
Bits 5:0 = ISR_[5:0]: These bits define the 64-
Kbyte memory segment (among 64) which con-
tains the interrupt vector table and the code for in-
terrupt service routines and DMA transfers (when
the PS bit of the DAPR register is reset). These
bits are used as the most significant address bits
(A21-16). The ISR is used to extend the address
space in two cases:
To generate the 22-bit Program memory address,
the contents of the CSR register is directly used as
the 6 MSBs, and the 16-bit virtual address as the
16 LSBs.
Note: The CSR register should only be read and
not written for data operations (there are some ex-
ceptions which are documented in the following
paragraph). It is, however, modified either directly
by means of the jps and calls instructions, or
indirectly via the stack, by means of the retsin-
struction.
– Whenever an interrupt occurs: ISR points to the
64-Kbyte memory segment containing the inter-
rupt vector table and the interrupt service routine
code. See also the Interrupts chapter.
– During DMA transactions between the peripheral
and memory when the PS bit of the DAPR regis-
ter is reset : ISR points to the 64 K-byte Memory
segment that will be involved in the DMA trans-
action.
CODE SEGMENT REGISTER (CSR)
R244 - Read/Write
Register Page: 21
Reset value: 0000 0000 (00h)
7
0
0
2.7.4 DMASR: DMA Segment Register
DMA SEGMENT REGISTER (DMASR)
R249 - Read/Write
Register Page: 21
Reset value: undefined
0
CSR_5 CSR_4 CSR_3 CSR_2 CSR_1 CSR_0
Bits 7:6 = Reserved, keep in reset state.
7
0
0
Bits 5:0 = CSR_[5:0]: These bits define the 64-
Kbyte memory segment (among 64) which con-
tains the code being executed. These bits are
used as the most significant address bits (A21-16).
DMA
DMA
DMA
DMA
DMA
DMA
0
SR_5 SR_4 SR_3 SR_2 SR_1 SR_0
Bits 7:6 = Reserved, keep in reset state.
2.7.3 ISR: Interrupt Segment Register
INTERRUPT SEGMENT REGISTER (ISR)
R248 - Read/Write
Bits 5:0 = DMASR_[5:0]: These bits define the 64-
Kbyte Memory segment (among 64) used when a
DMA transaction is performed between the periph-
eral's data register and Memory, with the PS bit of
the DAPR register set. These bits are used as the
most significant address bits (A21-16). If the PS bit
is reset, the ISR register is used to extend the ad-
dress.
Register Page: 21
Reset value: undefined
7
0
0
0
ISR_5 ISR_4 ISR_3 ISR_2 ISR_1 ISR_0
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STMICROELECTRONICS [ ST ]
