Execute Brain****: Difference between revisions
→{{header|Fortran}}: Ah yes, a further idea.
(→{{header|Fortran}}: Some details clarified.) |
(→{{header|Fortran}}: Ah yes, a further idea.) |
||
Line 955:
=={{header|Fortran}}==
Initial puzzlement as to the nature of the scratchpad was resolved: the source code being interpreted is in one storage area and the data scratchpad is another. Thus, self-modifying code is ''not'' in fact possible, so higher level of brain**** is precluded - as are still further opportunities offered by having the instruction and data pointers being in the data scratchpad rather than as separate items. Later experimentation showed that the initial value of all the STORE cells must be zero.
The source employs F90 so as to gain the convenience of a service routine SEEK contained within RUN that thereby has access to the PROG and the instruction pointer - though these could have been passed as additional parameters. The main idea is that the expression can fit on one line and special code is not used for the two cases. The STORE array of cells is represented as an array of CHARACTER*1 variables rather than a CHARACTER*n single variable. This means that an element is addressed as STORE(i), rather than STORE(i:i), and that STORE = CHAR(0) initialises the whole array to zero. If it were CHARACTER*n, then only the first character would be zero, all subsequent would be blanks. It is not clear what size a cell represents, but a single character suffices for the trial run. For usage that involves arithmetic, the ICHAR and CHAR functions are needed which work on values of 0:255. The cell array could be declared INTEGER*1 instead, which would allow arithmetic without sacrifices on the altar of type checking. Such a variable in two's complement has values of -128:127 however with only addition and subtraction supported this doesn't matter - the bit patterns are the same as for unsigned integers. Larger integer sizes are possible if required, but would require changes to the READ and WRITE statements because A1 format works at the high-order end of a multi-byte variable.
| |||