EET 2261 Unit 2 HCS12 Architecture

1. EET 2261 Unit 2HCS12 Architecture • Read Almy, Chapters 3, 4, and 5. • Homework #2 and Lab #2 due next week. • Quiz next week.

2. Programming Model • Inside any processor’s CPU are certain registers that the programmer has access to. • The list of these registers is often called the programming model. • For the HCS12:

3. HCS12 Programming Model • See page 30 of textbook or page 375 of HCS12 CPU Reference Manual.

4. Purposes of the Registers • Accumulators A, B, and D are general-purpose registers that the programmer can use however she wants. • PC and CCR are special-purpose registers that always perform specific functions: • PC always holds the address of the next instruction to be executed. • CCR always holds three mask bits and five status flags. • The other registers (X, Y, and SP) sometimes serve as general-purpose registers and sometimes perform specific functions.

5. Accumulators A, B, and D • The HCS12 has two 8-bit accumulators, named A and B. These two can also be regarded as forming a single 16-bit accumulator named D. • Some instructions operate on Accumulator A or Accumulator B. Other instructions operate on Accumulator D. • Examples: • LDAA loads an 8-bit number into A. • LDAB loads an 8-bit number into B. • LDD loads a 16-bit number into D.

6. HCS12 Instruction Set • A processor’s instruction set is the list of instructions (or commands) that the CPU recognizes. • Most of these instructions manipulate the CPU’s registers or locations in memory (or both).

7. Instruction Set Summary • For a summary of the HCS12’s instruction set, see the table on pages 381-394 of the Reference Manual.

8. Example: ABA • This table uses many abbreviations and special symbols. They’re all explained in the pages before the table (pages 377-380 of the Reference Manual.) • Let’s look at one instruction, ABA, which adds Accumulator B to Accumulator A and puts the results in Accumulator A.

9. First Column: Source Form • This column shows how you’ll type the instruction when you start writing assembly language programs next week. • ABA is very simple. Other instructions are more complicated and have additional things you need to type, explained here:

10. Second Column: Operation • This column explainsthe operation that the instruction performs. • Some symbols used in this column:

11. Third Column: Addressing Mode • This column shows the addressingmode(s) used by the instruction. As we’ll see below, there aresix different addressing modes. • Some abbreviations used in this column:

12. Fourth Column: Machine Coding • Computers use numbers torepresent all kinds of information, including the instructions in a program. This column shows the numbers (in hex) that represent each instruction. • Some abbreviations used in this column:

13. Fifth Column: Access Detail • This column shows howmany cycles each instruction takes to execute, and what happens during each of those cycles. • Some abbreviations used in this column:

14. Last Two Columns: Condition Codes • These columns show howeach instruction affects the bits in the Condition Code Register. • Symbols used in these columns:

15. Summary • This table summarizes a lot of useful information for every one of the 207 instructions that you can use when writing programs for the HCS12. • You’ll refer to this table often.

16. Categories of Instructions • The Instruction Set Summary table lists instructions alphabetically. Sometimes it’s more useful to have instructions grouped into categories of similar instructions. • Examples of categories: • Load and Store Instructions • Addition and Subtraction Instructions • Boolean Logic Instructions • … • See Section 5 (starting on p. 55) of the HCS12 CPU Reference Manual.

17. Instruction Set Details • For details on all 207 of the HCS12’s instructions, see pages 100-309 of the Reference Manual.

18. Mnemonics • In this course we’ll write programs in a language called assembly language. • Assembly language contains many mnemonics, which are abbreviations for actions that we want to perform. (Mnemonics appear in the Source Form column of the Instruction Set Summary.) • Some examples:

19. Operands • Some HCS12 instructions require the programmer to specify one or two operands, in addition to the mnemonic. • These operands are the data (usually numbers) to be operated on. • Example: • The LDAA instruction loads some number into accumulator A. The number that’s to be loaded is the operand. • In the instruction LDAA #5, the operand is 5.

20. Opcodes • When an instruction is translated into machine code, the mnemonic is replaced by an opcode. • Example: The assembly-language instruction LDAA #5becomes $8605 in machine code, because $86 is the opcode for LDAA. Mnemonic Operand Opcode

21. Review: Memory Addresses • Every computer system has memory, where programs and data are stored. • Each location within this memory has an address by which we identify it. And each location holds some contents, typically one byte. • The HCS12 uses a 16-bit address bus. • So the addresses of its memory locations range from $0000 to $FFFF. (In decimal, that’s 0 to 65,535.) • So we have 65,536 memory locations, each of which holds one byte.

22. Memory Map • Programmers must pay attention to where in memory their programs and data are stored. • Some of a system’s memory is reserved for special purposes. Therefore, within the range of possible addresses, only some can be used by your programs. • A memory map shows which memory addresses are reserved, and which are available for your use. • Next slide shows memory map for our HCS12 chip.

23. HCS12 Memory Map • See page 26 of Device User Guide. • Reserved addresses: • $0000 to $03FF are reserved for special-function registers. • $FF00 to $FFFF are reserved for interrupt vectors.

24. Addressing Modes • Every instance of an instruction uses an addressing mode. • The HCS12’s six addressing modes are: • Inherent • Immediate • Direct • Extended • Relative • Indexed(which has several variations)

25. Which Addressing Mode(s) Does an Instruction Use? • Some instructions come in only one “flavor”: they can only use one addressing mode. • Example: The ABA instruction always uses the inherent addressing mode. • Other instructions come in two or more “flavors.” • Example: The LDAA instruction can use immediate, direct, extended, or indexed addressing.

26. Inherent Addressing Mode • In inherent addressing mode, the programmer doesn’t need to specify any operands, because the mnemonic itself contains all of the information needed. • Example: The ABA instruction adds the numbers in Accumulators A and B. So we already know where the two operands are located, and we don’t need to say any more. • We just write ABA, instead of writing something like ABA #5.

27. Immediate Addressing Mode • In immediate addressing mode, the programmer gives the operand(s) as part of the instruction. • The pound sign (#) indicates immediate addressing. • Example: LDAA #15 causes the number 15 to be loaded into Accumulator A. • The following are all equivalent: • LDAA #15 • LDAA #$F • LDAA #%1111

28. Direct Addressing Mode • In direct addressing mode, the programmer gives the operand’s address as part of the instruction. • Example: LDAA 15 causes the number located at memory address 15 to be loaded into Accumulator A. • The following are all equivalent: • LDAA 15 • LDAA $F • LDAA %1111

29. Limitation of Direct Addressing Mode • In direct addressing mode, the operand’s address can only be one byte long. • So the highest address we can use with direct addressing is $FF, or 255. • Since there are 65,536 locations in memory, this means that 65,281 of those locations can’t be reached by direct addressing! • Solution: use extended addressing mode instead of direct addressing mode. (See next slide.)

30. Extended Addressing Mode • In extended addressing, the programmer gives the operand’s address as part of the instruction. But this address is two bytes long, so we can reach any memory location. • Example: LDAA $701F causes the number located at memory address $701F to be loaded into Accumulator A. • The following are all equivalent: • LDAA 28703 • LDAA $701F • LDAA %0111000000011111

31. Summary: Addressing Modes • We’ve discussed the first four of the HCS12’s six addressing modes: • Inherent • Immediate • Direct • Extended • Relative • Indexed • We’ll discuss the other two modes in the weeks ahead.

32. Condition Code Register • The Condition Code Register (CCR) is an 8-bit register that contains: • 3 mask bits (S, X, and I) • 5 flag bits (H, N, Z, V, C)

33. Meanings of the Flag Bits • The flag bits are set or reset based on the results of previously executed instructions. • H=1 means a half-carry (carry from bit 3 to bit 4) occurred. • N=1 means the result was negative (MSB=1). • Z=1 means the result was zero. • V=1 means a two’s-complement overflow occurred. • C=1 means a carry out of the MSB occurred. • But only some instructions affect these flag bits: check the instruction summary table.

34. Review: Two’s-Complement Notation • Recall that two’s-complement notation is the standard way of representing signed numbers in binary. • In this notation, the leftmost bit (MSB) is the sign bit. • Sign bit = 0 for positive numbers or 0. • Sign bit = 1 for negative numbers. • Examples: • %0000 0011 = 3 • %1000 0011 = 125

35. Meanings of the N and V Flag Bits • N=1 simply means that the result’s leftmost bit (MSB) was 1. If you’re using two’s-complement notation, this means that the result was negative. • V=1 means that a two’s-complement overflow occurred. Some examples: • We added two positive numbers (MSB=0) and got a negative result (MSB=1). • We added two negative numbers (MSB=1) and got a positive result (MSB=0).