Lecture XXI: I/O (1) Xuan Guo. CSC 3210 Computer Organization and Programming Georgia State University. April 7, Xuan Guo. Lecture XXI: I/O (1)

Transcription

1 CSC 3210 Computer Organization and Programming Georgia State University April 7, 2015

2 This lecture Plan for the lecture: External Data and Text printf function

3 External Data and Text Two external variables (data) non-zero initialized variables zero initialized variables When a program is loaded into memory, the program (text), variables are loaded into different regions of memory --- section text section data section bss section

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5 The text Section The text section is where the program and any read-only data are located. read-only pseudo-op Example:.section.text define(i_r, l0).global main main: save %sp, -96, %sp mov 1, %g1 ta 0

6 The data Section (1) The data section is for initialized data read-write pseudo-ops.section.data! the beginning of data section.word 3, 3*3, 3*3>>3! create word type data separate by comma.half 3! halfword type 3.byte 7! byte data 7.align n! align the external data

7 The data Section (2) ASCII Data & String.ascii hello! no zero at end.asciz hello! with a zero at end label data.section.data.align 4 i_m:.word 3 j_m:.word 3+9 fmt:.asciz hello, world\n

8 The.bss Section In the bss section, variables will be initialized to zero pseudo-op ary:.skip 4*100! provide 400 bytes 0 Example:.section.align 4.bss ary:.skip 4*100 i_m:.skip 4

9 sethi Instruction (1) The syntax looks like this: sethi const22, %reg where "const22" is a 22-bit integer constant (signed or unsigned is not relevant). It places the constant into the high-order 22 bits of the register, and sets the low-order 10 bits of the register to 0's. (?!) For example, sethi 0x333333,%l1! 0x is would set register %l1 to SPARC Instruction

10 sethi Instruction (2) Q: Why would you want to do this? A: In order to load a 32-bit constant (such as an address) into a register. This can't possibly be done in one instruction (since all instructions are 32 bits long, there isn't room for a 32-bit constant and also an opcode and a register number). There are instructions that can set the lower part of a register (add, or, etc), so this one complements those nicely. For example, to set %l1 to 0x89ABCDEF, do the following: 1. Split up 0x89ABCDEF into the top 22 bits and the bottom 10 bits 89ABCDEF = Top 22 bits are = 226AF3 Low 10 bits are = 1EF 2. Place the two halves into %l1 using separate instructions: sethi 0x226AF3,%l1 or %l1,0x1ef,%l1!or is better than add. (WHY?) SPARC Instruction

11 sethi Instruction (3) Shortcut #1: The SPARC assembler provides two special "functions" to make this easier. %hi(x) will give the top 22 bits of the constant X and %lo(x) will give the bottom 10 bits. This is an assembler feature. It is not part of the SPARC machine language. So we could use sethi %hi(0x89abcdef), %l1 or %l1,%lo(0x89abcdef), %l1 The most common use of this instruction is to place the address of something into a register. For example, if there is a character string in memory with the label "Prompt" on it, you can put the address of that string into %o1 using sethi or %hi(prompt),%o1 %o1,%lo(prompt),%o1 SPARC Instruction

12 set Instruction The above pair of instructions is used quite a lot, so the assembler provides a "synthetic instruction" which will generate them for you. The "instruction" set const32, %reg will accept any 32-bit constant (const32) such as an address, and any register (%reg) and will generate sethi or %hi(const32),%reg %reg,%lo(const32),%reg However, it should be remembered that SET is not a real SPARC instruction, and that it produces two machine language instructions, not one. SPARC Instruction

13 I/O on SPARC I/O thru buffers allocated for each terminal is an tremendous work in assembly. In SPARC, using system call printf and scanf is an easy way to perform I/O These functions perform conversion and extraction automatically. Your data could be resided in any memory block (.data,.bss, or stack for stack)

14 printf To use printf function, at least the following must be satisfied: %o0 must be containing the address of a null terminated string (cstring). A null terminated string is a string ended with 1 byte zero. Example: mystring:.asciz hello world!...!in your data section! and in your code set mystring, %o0! Get the address of the label call printf! this should print hello world! nop

15 printf cont. printf allows you to substitute some other data into your output. If your string pointed by %o0 contains %F, you must have your register %o1-%o5 containing the values to be substituted in the same order as the %F (except string), where F is one of the following 1. d or i Decimal signed integer. 2. o Octal integer. 3. x or X Hex integer. 4. U Unsigned integer. 5. c Character. 6. s C-String, i.e. null terminated string. 7. f Double 8. e or E Double. 9. g or G Double. 10. n Number of characters written by this printf.

16 Output formats o 0 prefix inserted. x or X values. (Hex) 0x prefix added to non-zero e or E Always show the decimal point. f Always show the decimal point. g or G Always show the decimal point trailing zeros not removed.

17 printf examples!assuming your string declared as follow mystring:.asciz it s %d dollars!in your code set mystring, %o0! The address of the string call printf!will print it s 20 dollars mov 20, %o1!substitutes for the first %d

18 printf examples cont.!assuming your string declared as follow mystring:.asciz it s %d dollars and %d cents!in your code set mystring, %o0! The address of the string mov 20, %o1! substituted for the first %d call printf! Will print it s 20 dollars and 5 cents mov 5, %o2! substituted for the second %d

19 Printing a string To use printf to print a string, the address of the substituted string must be in the correspondent register. Example:!assuming your strings are in data section as follows mystring:.asciz Hello %s! name:.asciz Michael...! and your code should be set mystring, %o0! address of the output set name, %o1! Address of the substituted string call printf! Will print Hello Michael! nop