ECE 265 – Lecture 3

1. ECE265 ECE 265 – Lecture 3 68HC11 Address Space, Memory, Registers, and data transfers

2. Lecture Overview • How is the address space broken out in the 68HC11 • RAM and ROM memories • Structure and Logical organization memory • Registers • Material from Chapter 2 and a 68HC11 reference manual ECE265

3. What is the difference – RAM and ROM • RAM = Random Access Memory ECE265

4. What is the difference – RAM and ROM • RAM = Random Access Memory • As the name implies any location of the memory can be accessed in any order, i.e., randomly. • Given an address, the data is stored at that address, or the data at that address is retrieved, depending on the mode of access (read or write). • RAM is the memory where data is stored. • ROM = Read Only Memory • Memory that can only be read. ECE265

5. What is the difference – RAM and ROM • RAM = Random Access Memory • As the name implies any location of the memory can be accessed in any order, i.e., randomly. • This is the memory where data is stored. • ROM = Read Only Memory • This is memory that can only be read. • There are different types • One time programmable, UV erasable, EEPROM – electrically erasable • ROM will maintain the contents even when power is off. ECE265

6. Different flavors of the 68HC11 • Different flavors of the chip have different amounts • MC68HC11A8 – 256 bytes of RAM and 8K of ROM • MC68HC11E9 – 512 bytes RAM, 12K ROM • MC68HC11D3 – 192 bytes RAM, 4K ROM • The different amounts allow choice of a chip tailored to the embedded system. • Note: There is also a 192 byte ROM on board for use in testing that has a bootstrap loading program. This ROM is also used for security. ECE265

7. The RAM • The entire memory space – 16 bit address (65,536) • RAM is in the first 256 loactions - $0000 to $00FF • These locations are available using direct addressing mode. • Direct addressing mode assumes the upper byte of the address is $00 • Saves space in program memory – only 8 bits for address and a 1 word instruction versus 2 • Saves a cycle of execution as only one fetch to load the instruction • By using the INIT register, 4-bits, RAM can be moved to the start of any 4K partition within the memory space as the INIT register specifies the upper 4-bits when direct addressing modes is used. INIT starts with 0000 ECE265

8. Power and RAM • The contents in RAM are lost if power is lost. • The design is fully static RAM so the data is not lost if the clocks are halted. • In DRAM – Dynamic RAM – clocks are needed to keep the data refreshed as the contents are not static. The charge that determines the value of that bit will bleed off if not refreshed. ECE265

9. The memory map • The complete 64K memory space • Note that EEPROM is located at $B600 to $B7FF • The map also supports add-on RAM – another chip in the system. • There is also a monitor EPROM ECE265

10. The Registers • Within the CPU • The 68HC11 is an accumulator based architecture • What is an accumulator based architecture? • First consider the programmers model of the architecture. • The programmers model is what the programmer see, i.e., what the programmer has direct access to. Allows the programmer to visualize how data is transferred between registers. • In may cases there are other registers within a processor architecture that required for operation but are not directly visible to the programmer. An example is the Instruction Register. ECE265

11. The Registers • Within the CPU • The 68HC11 is an accumulator based architecture • What is an accumulator based architecture? • An architecture in which all data operations require the data to be loaded in or interact with the accumulator. • First consider the programmers model of the architecture. • The programmers model is what the programmer see, i.e., what the programmer has direct access to. Allows the programmer to visualize how data is transferred between registers. • In most cases, there are other registers within a processor architecture that are required for operation but are not directly visible to the programmer. An example is the Instruction Register. ECE265

12. The 68HC11 Programmer Model • 7 registers in total • 2 are index registers • Used in addressing data • A stack pointer register • The PC • A CC register • The accumulator ECE265

13. The Accumulator • Actually have 2–Accumulator A &Accumulator B • Can be combined to form Accumulator D • For the most part they can be used to perform the same operations. • EXCEPT – • Decimal arithmetic – BDC operation must use A • CC register can only be transferred to A • Certain operations put the result in A ECE265

14. Example of Accumulator Use • The ABA instruction • Add accumulators • Result is loaded into Accumulator A ECE265

15. Accumulator D • Can perform a Load Double Accumulator Instruction • Loads both the A and B accumulators from Memory • Can also perform add and subtract operations on D • Can use the D accumulator in shift operations • The D accumulator is not a separate accumulator • If you use the D accumulator for an operation it will replace the contents of the A and B accumulators as it is the same physical register. ECE265

16. Lecture summary • Have covered • The pin configuration on the M68HC11 • The organization of the pins • The basic interface capabilities of the pins ECE265

17. Assignment • Read Chapter 2 • Look at Chapter 2, Problems: 2, 4, 5, 6, 7, 8, 9, 10 ECE265