Write a program in ARM assembly language (which can be executed on ARMSim simula

Question

Write a program in ARM assembly language (which can be executed on ARMSim simulator) to convert an ASCII string containing positive or negative integer decimal string to an integer. Input is given as null-terminated string stored at location given by the label value. The string can containing an optional '+' or '' followed by some combination of the digits 0 through 9. Your program should compute the integer value equivalent to this string of digits, then place the number in register R1. If a non-digit character appears anywhere in the string, your program should stop with the value 1 in register R1. For example, if input is "24" (or in ASCII 0x32, 0x34, 0x00) then register R1 should contain the value 24ten. (Hint: The ARM MUL instruction takes two registers as input. There is no "MULI" instruction. Thus, just store the constant 10 in a register.)

Explanation / Answer

A program's use of internal memory data may have to be restricted to read-only values. If you are writing a program which might one day be stored in a ROM, rather than being loaded into RAM, you must bear in mind that performing an instruction such as:

STR R0, label

will not have the desired effect if the program is executing in ROM. So, you must limit internal references to look-up tables etc. if you wish your code to be ROMmable. For example, the BBC BASIC interpreter only accesses locations internal to the program when performing tasks such as reading the tables of keywords or help information.

A related restriction on ROM code is that it should not contain any self-modifying instructions. Self-modifying code is sometimes used to alter an instruction just before it is executed, for example to perform some complex branch operation. Such techniques are regarded as bad practice, and something to be avoided, even in RAM programs. Obviously if you are tempted to write self-modifying code, you will have to cope with some pretty obscure bugs if the program is ever ROMmed.

Finally, the need for position-independence is an important consideration when you write code for ROM. A ROM chip may be fitted at any address in the ROM address space of the machine, and should still be expected to work.

The only time it is safe to write to the program area is in programs which will always, always, be RAM-based, e.g. small utilities to be loaded from disc. In fact, even RAM-based programs aren't entirely immune from this problem. The MEMC memory controller chip which is used in many ARM systems has the ability to make an area of memory 'read-only'. This is to protect the program from over-writing itself, or other programs in a multi-tasking system. Attempting to write to such a region will lead to an abort, as described in Chapter Seven.

It is a good idea, then, to only use RAM which has been allocated explicitly as workspace by the operating system, and treat the program area as 'read-only'.

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