ECE 353 Introduction to Microprocessor Systems

1. ECE 353Introduction to Microprocessor Systems Michael G. Morrow, P.E. Week 4

2. Data Transfers and Addresses Addressing Modes Memory Allocation Structures Strings Table Look-up Address Object Transfers Memory Alignment Considerations Objectives

3. Data Transfers • Internal vs. External Registers • All registers in the processor have addresses – they’re just not all in the same space. • Logical Addresses • Registers • I/O • Memory • Addressing Modes • Addressing modes determine how and when operands can be accessed in registers, I/O space, and memory space.

4. Instructions of the Week • IN acc, port • OUT port, acc • INS dst_s, port • OUTS port, src_s • MOV dst, src • XCHG dst, src • MOVS dst_s, src_s • LODS src_s • STOS dst_s • XLAT translate_tble

5. Addressing Modes • Immediate Addressing • The operand is encoded within the instruction itself • Immediate operands are constants, so they must be the source operand, not the destination • Sign extended as required • ASM86 numeric notation suffixes – B, Q or O, H • Defined constants (EQU) • Character constants • Assembler expressions • Allow assembler to do some low level calculations • Conditional assembly • MOV instruction

6. Addressing Modes • Register Addressing • The operand is one of the 80C188EB’s internal registers. • Register operand encoding only requires 3-bits plus w bit, so shorter instructions. • Does not run an external bus cycle • EA calculation • R/W cycle • XCHG Instruction

7. Addressing Modes • I/O Port Addressing • IN and OUT are most common I/O instructions. • AX or AL must be used as the source (OUT) or destination (IN). • Width of transfer determined by register choice. • Physical address always has A19:16 = 0. • Segment registers are not used.

8. Addressing Modes • Fixed Port Addressing (I/O) • Address of I/O port is encoded within instruction, limited to 8-bit addresses • Can only access 0000h – 00FFh • Can be used for byte or word ports • 80C188EB requires two transfers to read/write word ports • I/O port address is constant, i.e. must be known at assembly-time • IN instruction

9. Addressing Modes • Variable Port Addressing (I/O ) • DX register is used to supply a 16-bit address • Can access entire I/O space • Byte or word transfers • DX is not modified by instruction, just used as a pointer • Allows for port addresses to be computed or operated on at run-time (i.e. incrementing through a range of port addresses in a loop)

10. Addressing Modes • I/O String Port Addressing • INS • Transfers from an I/O port to ES:DI • OUTS • Transfers from DS:SI to an I/O port • Requires two transfers, i.e. INS transfers from I/O port to a temporary register, then the temporary register is transferred to memory • Most often used in combination with the REP modifier for low-overhead block moves

11. Memory Allocation • Memory operand types • Byte, word, double-word • Stored in little-endian format • Data item attributes • Type, Offset, Segment • Data allocation directives • DB, DW, DD • Identifiers • Initializers • Arrays and strings • Setting up a data segment • Variable naming

12. Memory Addressing Modes • Every memory location is referred to by a logical address (segment:offset) • BIU provides the segment • EU supplies the offset (EA) • EA calculation is based on the addressing mode • In general, the EA is computed as: • EA = [displacement] + [BX or BP] + [SI or DI] • Displacement is an 8- or 16-bit constant supplied at assembly-time • EA computation can take significant part of instruction execution time

13. Memory Addressing Modes • Direct Addressing • EA is a variable’s offset in a given segment • EA is contained in the displacement field of instruction (16-bit value) • The address is a constant determined at assembly-time • The segment register used is based on the instruction (default segment) or the use of a segment override prefix

14. Memory Addressing Modes • Register Indirect Addressing • EA is contained in an index or pointer register • BX, BP, DI , SI • Remember – BP always uses SS • Getting the EA of a label • The OFFSET assembler operator. • Resolving anonymous memory references • The PTR assembler operator • Other assembler attribute operators

15. Memory Addressing Modes • Indexed Addressing • EA = displacement + [SI, DI, BX or BP]. • Displacement is a constant calculated at assembly-time. • Typically used for accessing data in arrays. • Displacement = array starting offset • Register holds (element index × element size) • If array of bytes, size = 1 • If array of words, size = 2 • If array of double-words, size = 4

16. Memory Addressing Modes • Based Addressing • EA = [BX, BP, DI or SI] + displacement. • Displacement is a constant calculated at assembly-time. • Typically used for accessing information in data structures. • Register holds starting address of structure. • Displacement = offset from start of structure to desired structure element. • Code can access any instance of the structure just by changing register contents to point at it.

17. Memory Addressing Modes • Based Indexed Addressing • EA = [BX, BP] + [DI, SI] + displacement. • Displacement is a constant calculated at assembly-time. • Typically used for accessing information in arrays of data structures. • BX = starting offset of array • DI = offset to desired array element • Displacement = offset within a structure to the desired element.

18. Memory Addressing Modes • String Addressing • Memory source is DS:SI • Can override to CS, ES, SS • Memory destination is ES:DI • String instruction primitives • INS, OUTS, MOVS, LODS, STOS • The size of the operand is determined by the data type or by appending B or W to the mnemonic. • All automatically update DI/SI based on the direction flag (DF) and operand size. • String instructions commonly use the REP prefix to do block transfers (more on that later)

19. Creating Data Structures • Structure template • STRUC / ENDS • Defines a data structure, does not allocate memory. • Structure use • Allocating space for structure. • Initializing structure members. • Referring to structure members using direct addressing. • Creating and indexing arrays of structures.

20. Addressability • Operand addressability • Definition • Using the ASSUME directive • Purpose • Relation to segment register contents • Segment override prefixes • Assembler generated • Explicit

21. Allocating ROM Space • In an embedded system, the code segment is loaded into ROM. • Data variables in the code segment will be placed in ROM also – so are constant. • Example • Look-up tables • Purpose • Usage • Indexed addressing • XLAT instruction

22. Address Object Transfers • Used to load an address into a register for indirect addressing. • LEA • Calculates the EA for the source operand and stores it in the destination register. • LDS / LES • Loads from a double-word in memory pointed to by the source operand (direct/indirect/etc.). • Low-word is transferred to destination register (often SI/DI), high-word is transferred to the segment register (DS/ES).

23. 80C186EB Alignment Issues • The 80C186EB has a 16-bit data bus. • Loading a word only takes one cycle, if the word is properly aligned. If loading an unaligned word, two bus cycles are required. • 80C186EB memory map • Data alignment is often an important factor to maximize the efficiency of a wide data bus. • EVEN assembler directive • Forces the following data to be located on a word boundary (even address). • Assembler will pad with a NOP to correct alignment.

24. In-Class Exercises • Create a source code template with a code segment and a data segment, establish addressability • Declare a 100 byte array in DS and initialize to 0 • Create a far (32-bit) pointer to the array in DS • Declare a word variable in CS • Copy the segment part of the far pointer to the word variable you just declared • Load BL with the 51st element of the array using indexed addressing • Fill the 99th array element with its index using based addressing • Exchange the 99th and 100th elements of the array • XCHG instruction

25. Wrapping Up

26. MOV instruction

27. XCHG Instruction

28. IN Instruction

29. OUTS instruction

30. Hungarian Notation c signed character uc unsigned character i integer ui unsigned integer si short integer li long integer n an integer number where the actual size is irrelevant f float d double s string of characters sz string of characters, terminated by a null character b an integer or character used as a boolean value by single byte ct an integer being used as a counter or tally p pointer to a structure or general void pointer px pointer to a variable of class x, e.g. pi, pf, pli

31. ROM Variables .186 assume ds:data, cs:code data segment DATA_VAR db 1 ;declare DS variable data ends code segment CODE_VAR db 2 ;declare CS variable start: mov ax, data ;setup data segment mov ds, ax mov al, DATA_VAR;load DS variable mov al, CODE_VAR;load CS variable jmp start code ends end start

32. ROM Variable Listing … … 0006 A0 0000r mov al, DATA_VAR 0009 2E:A0 0000r mov al, CODE_VAR … … • 2Eh is the CS segment override prefix • r indicates that this is a relative offset – the linker will calculate the absolute value after it combines any other segment pieces into one segment.

33. Data Alignment • The 80C186 memory map is logically the same as the 80C188, but it is physically different. data segment Var1 dw 1234h db ? Var2 dw 789Ah data ends Var1 Var2

34. SelectedAssembler Expression Operators

35. XLAT Instruction