Samsung S3C2440A - Manual (Page 42)

S3C2440A RISC MICROPROCESSOR PROGRAMMER'S MODEL
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PROGRAMMER'S MODEL
OVERVIEW
S3C2440A is developed using the advanced ARM920T core, which has been designed by Advanced RISC
Machines, Ltd.
PROCESSOR OPERATING STATES
From the programmer's point of view, the ARM920T can be in one of the two states:
•
ARM state which executes 32-bit, word-aligned ARM instructions
•
THUMB state is a state which can execute 16-bit, halfword-aligned THUMB instructions. In this state, the PC
uses bit 1 to select between alternate halfwords
NOTES
Transition between these two states does not affect the processor mode or the contents of the registers.
SWITCHING STATE
Entering THUMB State
Entry into THUMB state can be achieved by executing a BX instruction with the state bit (bit 0) set in the operand
register.
Transition to THUMB state will also occur automatically on return from an exception (IRQ, FIQ, UNDEF, ABORT,
SWI etc.), if the exception is entered with the processor in THUMB state.
Entering ARM State
Entry into ARM state can be done by the following methods:-
• On execution of the BX instruction with the state bit clear in the operand register.
• On the processor taking an exception (IRQ, FIQ, RESET, UNDEF, ABORT, SWI etc.). In this case, the PC is
placed in the exception mode's link register, and execution commences at the exception's vector address.
MEMORY FORMATS
ARM920T views memory as a linear collection of bytes numbered upwards from zero. Bytes 0 to 3 hold the first
stored word, bytes 4 to 7 the second and so on. ARM920T can treat words in memory as being stored either in
Big-Endian or Little-Endian format.