Embedded Specific
8051 Memory Areas (cont.)
 Internal data memory
 Resides within the 8051 MCU and is read/write
 Up to 256 bytes of internal data is supported
 Very fast access because it is 8-bit addressable
 Types of internal data memory:
 data – the first 128 bytes, uses direct addressing
 idata – all 256 bytes, uses indirect addressing
 bdata – 16 bytes of bit addressable memory
 SFR memory
 128 bytes of memory for Special Function Registers
 Cannot be accessed indirectly
 History of development:
 8051 was designed to use 128 bytes of memory, was good for small devices
 To support compatibility with previous devices, new devices keep its architecture
but expanded it with new features
 This caused a variety of memory types and system complicity
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2. Embedded Specific
8051 Memory Areas (cont.)
 External Data Memory
 Read/write memory, up to 64 KByte
 Indirectly accessible
 Peripheral device may be mapped to this
memory space => memory-mapped I/O
 8051 memory types to access external
memory
 xdata refers to any location in the 64K Byte
address space of external data
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Processing Order: Cooperative vs. Preemptive
Cooperative Multitasking
Preemptive Multitasking
Process A
Process B1
Process A (low priority, cyclic)
Process B2
Process B (low priority)
OS
Process C
Process C (high priority, cyclic)
Process D
Process D (high priority)
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

Processes (tasks) are run in an
infinite loop
If task takes too much time (B), need
to divide it
No overhead occurs
Copyright © 2011 DSR Corporation
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
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Scheduling is performed by the OS
Long task is suspended if needed
Overhead: caused by OS internals
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