Loadstar 12

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Text File | 1985-01-01 | 6.8 KB | 356 lines

PEEKs, POKEs, and SYSes -- Part 28 -- continued from Part 27 -- Now, let's say we were to run this example program: 10 DIM AB(1,2),CD%(3),EF$(2) ====================================== CHART 3: What memory looks like after we run the example program: -------------------------------------- label description value addr -------------------------------------- TXTTAB--> BASIC program line 2049 ten. End of program. 2078 VARTAB--> 2079 ARYTAB--> variable name "a" 65 2079 variable name "b" 66 2080 +<-offset pointer(lo) 39 2081 ! offset pointer(hi) 0 2082 ! # of dimensions 2 2083 ! size of dim 2 (hi) 0 2084 ! size of dim 2 (lo) 3 2085 ! size of dim 1 (hi) 0 2086 ! size of dim 1 (lo) 2 2087 ! ab(0,0) - 5 bytes #0 2088 ! ab(1,0) - 5 bytes #0 2093 ! ab(0,1) - 5 bytes #0 2098 ! ab(1,1) - 5 bytes #0 2103 ! ab(0,2) - 5 bytes #0 2108 ! ab(1,2) - 5 bytes #0 2113 +->variable name "c" 195 2118 variable name "d" 196 2119 +<-offset pointer(lo) 15 2120 ! offset pointer(hi) 0 2121 ! # of dimensions 1 2122 ! size of dim 1 (hi) 0 2123 ! size of dim 1 (lo) 4 2124 ! cd%(0) - 2 bytes #0 2125 ! cd%(1) - 2 bytes #0 2127 ! cd%(2) - 2 bytes #0 2129 ! cd%(3) - 2 bytes #0 2131 +->variable name "e" 69 2133 variable name "f" 198 2134 +<-offset pointer(lo) 16 2135 ! offset pointer(hi) 0 2136 ! # of dimensions 1 2137 ! size of dim 1 (hi) 0 2138 ! size of dim 1 (lo) 3 2139 ! ef$(0) - 3 bytes "" 2140 ! ef$(1) - 3 bytes "" 2143 ! ef$(2) - 3 bytes "" 2146 STREND<+ 2149 ====================================== And now for the explanation. (Drum roll please...) Let's take it from the top. Our BASIC program starts at TXTTAB (address 2049) and extends to just before VARTAB (address 2079). VARTAB and ARYTAB point to the same place because our sample program used no simple variables. If we HAD used simple variables, they would be found somewhere between VARTAB and ARYTAB. THE FIRST ARRAY: AB The first array our program used was AB(1,2). So, the address pointed to by ARYTAB contains the name of our array. As you recall from our discussion of simple variables, the names of real variables have their high bits turned off. On our chart, at location 2079, 65 is the ASCII value of "A". At 2080 we find 66, the ASCII value of "B". The next two bytes of memory, 2081 and 2082, contain the offset to the address of the name of the next array. 2081 is the low byte and 2082 the high, so to find the address of the the SECOND array, add 39 (the contents of 2082) to 2079 (the address of the FIRST array). If you don't get 2118, try again. A glance at our chart shows that the name of the next array used (CD%) is, indeed, at 2118. But let's look at the FIRST array some more. Byte 2083 describes the number of dimensions. Its value is two because array AB had two subscripts. The next four bytes (2084 to 2087) tell us the size of each of AB's dimensions. It takes two bytes per dimension to describe the size of the array so the size of this block will vary. For array AB, 2084-85 tell us that the rightmost dimension has three elements (0, 1, and 2). 2086-87 describe the second dimension from the right. It has two elements (0 and 1). Starting at 2088, we find the actual values of the array variables. Array AB is a REAL array, so each variable is represented by five bytes. (You will remember from our discussion of simple variables that a real variable consists of an exponent byte followed by four mantissa bytes.) Arrays are stored with the rightmost index increasing most slowly. Look at locations 2088 to 2113 to see what I'm talking about. This array is two elements by three elements, so there are six variables reserved. Six variables times five bytes... hmmm... The next thirty bytes from 2088 will contain the values of all the variables in array AB. 2088 plus 30 is 2118. That's where our next array will start. You may remember that our offset pointer from the first array pointed to 2118. It turns out that array offset pointers always point to the next memory location after the end of their own array. THE SECOND ARRAY: CD% The second array our program used was called CD%, so we can expect to find the ASCII values for "C" and "D" in memory locations 2118 and 2119. You will recall from our discussion of simple variables that INTEGER names have the high bit set in both characters. 2120 and 2121 are the offset from second array to the third array. (2118 + 15 = 2133 ... That's where EF% should start.) 2122 holds the number of dimensions of array CD%. In this case, there was only one dimension. 2123 and 2124 describe the size of that single dimension -- 4 elements. The size here will always be one greater than the number used in the BASIC DIM statement, because element zero is always counted. The values of array CD% are stored starting at 2125. Because this is an INTEGER array, only two bytes are needed to hold the values. Unlike simple integers, subscripted integers do NOT waste three bytes of memory per variable. The values of the four variables in CD% are stored in the eight memory locations from 2125 to 2132. THE THIRD ARRAY: EF$ The first seven bytes of array EF$ are comparable to the first seven of CD% -- since both arrays have the same number of dimensions, both have the same amount of overhead. Locations 2133 and 2134 contain the name -- remember that only the SECOND byte of a STRING name has the high bit set. The offset pointer at 2135 and 2136 points to STREND. That means there are no more arrays after EF$. 2137 through 2139 describe the size of the array -- that stuff should be old hat to you by now. 2140 through 2148 contain the string pointers for the three strings of array EF$. As you remember from our discussion of simple variables, the values of string variables are not found with the variable name. Instead, strings are stored between FRETOP and MEMSIZ. The only information kept with a string's name is the length of the string and the address where its actual contents can be found. In string arrays, each element holds only these three bytes: Byte 0: The length of the string. Byte 1: The low byte of the string's address. Byte 2: The high byte of the string's address. ------< continued in Part 29 >--------