1. ECE 353 Introduction 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 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 51 st element of the array using indexed addressing Fill the 99th array element with its index using based addressing Exchange the 99 th and 100 th elements of the array XCHG instruction

25. Wrapping Up

26. MOV instruction

27. XCHG Instruction

28. IN Instruction

29. OUTS instruction

30. 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 Hungarian Notation

31. ROM Variables.186 assume ds:data, cs:code data segment DATA_VAR db 1;declare DS variable dataends codesegment CODE_VAR db 2 ;declare CS variable start:movax, data;setup data segment movds, ax moval, DATA_VAR;load DS variable mov al, CODE_VAR;load CS variable jmp start codeends endstart

32. ROM Variable Listing 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. … 0006 A0 0000r mov al, DATA_VAR 0009 2E:A0 0000r mov al, CODE_VAR …

33. Data Alignment The 80C186 memory map is logically the same as the 80C188, but it is physically different. LSBMSB DS:000034h12h DS:0002?9Ah DS:000478h DS:0006 DS:0008 DS:000A data segment Var1 dw 1234h db? Var2dw789Ah dataends Var2 Var1

34. Selected Assembler Expression Operators OperationSyntaxResult Arithmetic High byte isolationHIGH operandHigh byte of operand Low byte isolationLOW operandLow byte of operand Shift rightoperand SHR countBitwise right shift Shift leftoperand SHL countBitwise left shift Modulusoperand MOD operandRemainder Logical Logical ANDoperand AND operandBitwise AND Logical XORoperand XOR operandBitwise XOR Logical NOTNOT operandBitwise complement Relational Equaloperand EQ operand All 1’s if true, 0 otherwise Not equaloperand NE operand Less thanoperand LT operand

35. XLAT Instruction