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125 lines
4.5 KiB
Plaintext
125 lines
4.5 KiB
Plaintext
Pointer Functions
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This library contains functions for basic pointer access and manipulation.
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These functions are intended to allow sequential reading and writing of
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individual bytes to arbitrary locations in memory.
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Only one pointer may be active at a time, but it's contents may be saved
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and restored using the following code:
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savelo = ptrlo; savehi = ptrhi; //save current pointer
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ptrlo = savelo; ptrhi = savehi; //restore current pointer
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where savelo and savehi are an arbitrary pair of variables defined in the
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program.
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Note: There is no concept of a null pointer in C02. A pointer containing
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the value 0 simply points to the first byte of memory.
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In the equivalent C code examples below, the system pointer is represented
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as the variable p. In all cases, assume the following declaration :
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int *p;
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Usage: at the beginning of the program use the directives
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#include <pointer.h02>
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The following application functions are defined:
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ptrset(&v); Pointer Set: Set pointer contents to the address of
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variable v.
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This is equivalent to the C code
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p = &v;
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Note: Sets variables ptrlo and ptrhi.
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ptrput(b); Pointer Put: Stores value of b in the byte currently
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pointed to and increments the pointer.
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This is equivalent to the C code
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*p = b; p++;
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Note: Updates variables ptrlo and ptrhi.
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b = ptrget(); Pointer Get: Retrieves the contents of the byte
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currently pointed to and increments the pointer.
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This is equivalent to the C code
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b = *p; p++;
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Note: Updates variables ptrlo and ptrhi.
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ptrinc(); Pointer Increment: Increases the pointer value by 1,
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causing it to point to the next byte in memory.
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This is equivalent to the C code
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p++;
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Note: Updates variables ptrlo and ptrhi.
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ptrdec(); Pointer Decrement: Decreases the pointer value by 1,
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causing it to point to the previous byte in memory.
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This is equivalent to the C code
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p++;
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Note: Updates variables ptrlo and ptrhi.
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ptradd(n); Pointer Add: Adds the value n to the address contained
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in the pointer, moving it that many bytes forward in
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memory.
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This is equivalent to the C code
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p += n;
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Note: Updates variables ptrlo and ptrhi.
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ptrsub(n); Pointer Subtract: Adds the value n to the address
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contained in the pointer, moving it that many bytes
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backward in memory.
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This is equivalent to the C code
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p -= n;
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Note: Updates variables ptrlo and ptrhi.
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ptrcmp(&v); Pointer Compare: Compares pointer contents against
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the address of variable v.
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Returns 255 if the pointer is less than the address
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(pointing to a byte lower in memory), 0 if the pointer
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is equal to the address (pointing to the same byte),
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and 1 if greater than tge address (pointing to a
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byte higher in memory).
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These results can be evaluated using the C02
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unary comparator ! or the test-operators :+ or :-.
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This is equivalent to the C code
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if (p < &v) return 255;
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else if (p > &v) return 1;
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else return 0;
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Note: Sets variables srclo and srchi.
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Note: This library expects the following to be defined:
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the zero page variable pair
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ptrlo, ptrhi System Pointer
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and the transient variable
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temp0 Temporary storage
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