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Using the Assembler

Using the Assembler. Chapter 4 Operators and Expressions JMP and LOOP Instructions Indirect Addressing Using a Link Library. Producing the .lst and .map files. With MASM 6.11, the ML command assemble and links a .ASM file ml hello.asm It produces a .OBJ file and a .EXE file

graham-harding
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Using the Assembler

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  1. Using the Assembler Chapter 4 Operators and Expressions JMP and LOOP Instructions Indirect Addressing Using a Link Library

  2. Producing the .lst and .map files • With MASM 6.11, the ML command assemble and links a .ASM file • ml hello.asm • It produces a .OBJ file and a .EXE file • Option /Zi produces debugging info for CV • Option /Fl produces a source listing • Option /Fm produces a map file • Ex: to produce all of these, type (with spaces) • ml /Zi /Fl /Fm hello.asm

  3. Examining the .lst and .map files • hello.lst • The R suffix of an address indicates that it is relocatable (resolved at loading time) • With the .model small directive, MASM aligns the segments in memory in the following way: • the stack segment is align at the next available paragraph boundary (ie: at a physical address divisible by 10h) • the code and data segments are aligned at the next available word boundary (ie: at a physical address divisible by 2) • The (relative) starting physical address of the segments are found in the hello.map file

  4. Alignment of the data segment • The offset address of the first data is generally not 0 if the data segment is loaded on a word boundary • Ex: if the code part of hello.asm takes 11h bytes and starts at 30000h. The first data will start at 30012h and DS will contain 3001h. • .data • message db "Hello, world!",0dh,0ah,'$’ • The offset address of message will be 2 instead of 0 (check this with Code View) mov bx, offset message ; BX=2 mov bx, offset message+1 ; BX=3 ...

  5. Alignment of the data segment (cont.) • TASM however will align the data segment at the first paragraph boundary (with the .model small directive) • So the offset address of the first data in the data segment will always be 0 • (like it is indicated in section 4.1.4 of the textbook)

  6. Memory Models supported by MASM and TASM

  7. Processor Directives • By default MASM and TASM only enable the assembly of the 8086 instructions • The .386 directive enables the assembly of 386 instructions • instructions can then use 32-bit operands, including 32-bit registers • Place this directive just after .model • Same principle for all the x86 family • Ex: use the .586 directive to enable the assembly of Pentium instructions

  8. Using a Link Library • A link library is a file containing compiled procedures. Ex: irvine.lib contains procedures for doing I/O and string-to-number conversion (see table 5 and appendix e). Ex: • Readint : reads a signed decimal string from the keyboard and stores the corresponding 16-bit signed number into AX • Writeint_signed : displays the signed decimal string representing the signed number in AX • To use these procedures in intIO.asm : • ml irvine.lib intIO.asm

  9. The EXTRN directive • A pgm must use the EXTRN directive whenever it uses a name that is defined in another file. Ex. in intIO.asm we have: • extrn Readint:proc, Writeint_signed:proc • For externally defined variables we use either byte, word or dword. Ex: • extrn byteVar:byte, wordVar:word • For an externally defined constant, we use abs: • extrn true:abs, false:abs

  10. The LOOP instruction • The easiest way to repeat a block of statements a specific number of times • LOOP label • where the label must precede LOOP by less than 127 bytes of code • LOOP produces the sequence of events: • 1) CX is decremented by 1 • 2) IF (CX=0) THEN go to the instruction following LOOP, ELSE go to label • LOOPD uses the 32-bit ECX register as the loop counter (.386 directive and up)

  11. The LOOP instruction (cont.) • Ex: the following will print all the ASCII codes (starting with 00h): • mov cx,128 • mov dl,0 • mov ah,2 • next: • int 21h • inc dl • loop next • If CX would be initialized to zero: • after executing the block for the 1st time, CX would be decremented by 1 and thus contain FFFFh. • the loop would thus be repeated again 64K times!! Break ...

  12. Indirect Addressing • Up to now we have only used direct operands • such an operand is either the immediate value we want to use or a register/variable that contains the value we want to use • But to manipulate a set of values stored in a large array, we need an operand that can index (and run along) the array • An operand that contains the offset address of the data we want to use is called an indirect operand • To specify to the assembler that an operand is indirect, we enclose it between [ ]

  13. Indirect Addressing (cont.) • Ex: if the word located at offset 100h contains the value 1234h, the following will load AX with 1234h and SI=100h: mov ax,[si] ;AX=1234h if SI=100h • In contrast, the following loads AX with 100h: mov ax,si ;AX=100h if SI=100h • In conclusion: mov ax,si ;loads AX with the content of SI mov ax,[si] ;loads AX with the word pointed by SI

  14. Ex: summing the elements of an array • .data • Arr dw 12,26,43,13,97,16,73,41 • count = ($ - Arr)/2 ;number of elements • .code • mov ax,0 ;AX holds the sum • mov si,offset Arr • mov cx,count • L1: • add ax,[si] • add si,2 ;go to the next word • loop L1

  15. Indirect Addressing (cont.) • For 16-bit registers: • only BX, BP, SI, DI can be used as indirect operands • For 32-bit registers: • EAX, EBX, ECX, EDX, EBP, ESP, ESI, EDI can be used as indirect operands • Caution when using 32-bit registers in real mode (only the 1st MB is addressable): • mov ebx, 1000000h • mov ax, [ebx] ;outside real-mode address space

  16. Indirect Addressing (cont.) • The default segment used for the offset: • it is SS whenever BP, EBP or ESP is used • it is DS whenever the other registers are used • This can be overridden by the “:” operator: mov ax, [si] ;offset from DS mov ax, es:[si] ;offset from ES mov ax, [bp] ;offset from SS mov ax, cs:[bp] ;offset from CS • With indirect addressing, the type is adjust according to the destination operand: • mov ax,[edi] ;16-bit operand • mov ch,[ebx] ;8-bit operand • mov eax,[si] ;32-bit operand

  17. Base and Index Addressing • Base registers (BX and BP), index registers (SI and DI) and 32-bit registers can be use with displacements (ie: constant and/or variable) • If A is a variable, the following forms are permitted: mov ax, [bp+4] mov ax, 4[bp] ;same as above mov ax, [si+A] mov ax, A[si] ;same as above mov ax, A[edx+4]

  18. Base and Index Addressing (cont.) • Example of using displacements: • .data • A db 2,4,6,8,10 • .code • mov si,3 • mov dl, A[si+1] ;DL = 10

  19. Base-Index Addressing • Base-index addressing is used when both a base and an index register is used as an indirect operand. • When two 16-bit registers are used as indirect operands: the first one must be a base and the second one must be an index: mov ah, [bp+bx] ;invalid, both are base mov ah, [si+di] ;invalid, both are index mov ah, [bp+si] ;valid, segment is in SS mov ah, [bx+si] ;valid, segment is in DS

  20. Base-Index Addressing (cont.) • A two dimensional array example: • .data • rowsize = 3 • arr db 10h, 20h, 30h • db 0Ah, 0Bh, 0Ch • .code • mov bx, rowsize ;choose 2nd row • mov si, 2 ;choose 3rd column • mov al, arr[bx+si] ;AL = 0Ch • mov al, arr[bx][si]

  21. Base-Index Addressing with 32-bit registers • Both of the 32-bit registers can be base or index (previous restriction is lifted) • mov ax, [ecx+edx] ;permitted, both are index • mov ax, [ebx+edx] ;permitted, base and index • mox ax, [ebx][edx] ;same as above • The 1st register determines the segment used: • mov ax,[esi+ebp] ;offset from DS • mov ax,[ebp+esi] ;offset from SS • We can also add displacements • mov dh, A[esi][edi+2] Break ...

  22. The OFFSET Operator • The OFFSET returns the distance of a label or variable from the beginning of its segment. • Example: .data bList db 10h, 20h, 30h, 40h wList dw 1000h, 2000h, 3000h .code mov al, bList ; al = 10h mov di, offset bList ; di = 0000 mov bx, offset bList+1 ; bx = 0001

  23. The SEG Operator • The SEG operator returns the segment part of a label or variable’s address. • Example: push ds mov ax, seg array mov ds, ax mov bx, offset array . pop ds

  24. The PTR Operator (directive) • Sometimes the assembler cannot figure out the type of the operand. Ex: • mov [bx],1 • should value 01h be moved to the location pointed by BX, or should it be value 0001h ? • The PTR operator forces the type: • mov byte ptr [bx], 1 ;moves 01h • mov word ptr [bx], 1 ;moves 0001h • mov dword ptr [bx], 1 ;moves 00000001h

  25. The LABEL directive • It gives a name and a size to an existing storage location. It does not allocate storage. • It must be used in conjunction with byte, word, dword, qword... • .data • val16 label word • val32 dd 12345678h • .code • mov eax,val32 ;EAX = 12345678h • mov ax,val32 ;error • mov ax,val16 ;AX = 5678h • val16 is just an alias for the first two bytes of the storage location val32

  26. The TYPE Operator • It returns the size, in bytes, of a variable: • .data • var1 dw 1, 2, 3 • var2 dd 4, 5, 6 • .code • mov bx, type var1 ;BX = 2 • mov bx, type var2 ;BX = 4 • Handy for array processing. Ex: If SI points to an element of var2, then to make SI point to the next element, we can simply write: add si, type var2

  27. The LENGTH, SIZE Operators • The LENGTH operator counts the number of individual elements in a variable that has been defined using DUP. • .data • var1 dw 1000 • var2 db 10, 20, 30 • array dw 32 dup(0) • .code • mov ax, length var1 ; ax=1 • mov ax, length var2 ; ax=1 • mov ax, length array ; ax=32 • The SIZE operator is equivalent to LENGTH*TYPE

  28. Sign and Zero Extend Instructions • MOVZX (move with zero-extend) • MOVSX (move with sign-extend) • Both move the source into a destination of larger size (valid only for 386 and later processors) • imm operands are not allowed • mov bl, 07h • mov bh, 80h • movzx ax,bh ;AX = 0080h • movsx dx,bh ;DX = FF80h • movsx ax, bl ;AX = 0007h • movzx ecx,dx ;ECX = 0000FF80h • movsx ecx,dx ;ECX = FFFFFF80h • movsx ecx,ax ;ECX = 00000007h

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