Difference between revisions of "!"
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| [[BASIC metasyntax|Syntax]] | | [[BASIC metasyntax|Syntax]] | ||
| BASIC I-V | | BASIC I-V | ||
| − | | <num-var> = [< | + | | <num-var> = [<num-var>]<code>!</code><numeric><br>[<num-var>]<code>!</code><numeric> = <numeric> |
|- style="vertical-align:top" | |- style="vertical-align:top" | ||
| Token (hex) | | Token (hex) | ||
| Line 52: | Line 52: | ||
Whether unary or binary, the operator will either read or write to memory, | Whether unary or binary, the operator will either read or write to memory, | ||
depending on whether it is evaluated as an expression, or assigned a | depending on whether it is evaluated as an expression, or assigned a | ||
| − | value. In the latter case the whole [< | + | value. In the latter case the whole [<num-var>]<code>!</code><numeric> |
expression serves as an ''lvalue'', or <num-var>. | expression serves as an ''lvalue'', or <num-var>. | ||
| Line 78: | Line 78: | ||
The BBC Microcomputer User Guide is peppered with warnings about the use | The BBC Microcomputer User Guide is peppered with warnings about the use | ||
of <code>?</code>, <code>!</code> and <code>$</code>. They are not to be | of <code>?</code>, <code>!</code> and <code>$</code>. They are not to be | ||
| − | used to access memory-mapped devices or the system's internal variables | + | used to access memory-mapped devices or the system's internal variables – |
at least, not in published programs. The relevant addresses may change or | at least, not in published programs. The relevant addresses may change or | ||
disappear on different machines and MOS versions, or the program may find | disappear on different machines and MOS versions, or the program may find | ||
Revision as of 14:07, 8 March 2015
The exclamation mark, !, is a BASIC operator to read or change doublewords in memory. In this context it is pronounced 'pling', 'bang' or, rarely, 'shriek'. It is somewhat equivalent to PEEK and POKE statements in other dialects of BASIC.
| Availability | Present in all original versions of BBC BASIC. | |
| Syntax | BASIC I-V | <num-var> = [<num-var>]!<numeric>[<num-var>] !<numeric> = <numeric>
|
| Token (hex) | BASIC I-V | 21 (operator, lvalue)
|
| Description | BASIC I-V | If there are two operands, ! adds them to obtain an effective address. Otherwise, the single operand is the effective address.In the first form, ! returns the doubleword value at the effective address.In the second form, the value assigned to ! is written to the doubleword at the effective address.
|
| Associated keywords | ?, $, CALL, USR
| |
Description
! is an operator providing access to the memory of the
machine running BASIC. It allows the contents of memory to be inspected
or changed one doubleword at a time. A doubleword is a unit of
four bytes, and can hold a value between -2,147,483 648 and 2,147,483,647
or between &00000000 and &FFFFFFFF. By contrast the ?
operator acts on individual bytes.
Like - and ?, ! can be either a
unary or binary operator. This doesn't depend on whether it is
used to PEEK or POKE, but is a syntactic convenience to help with
handling data structures.
If used as a unary operator, for example !&3000 or
!osword_block%, then the single operand is used as the
effective address.
If used as a binary operator, for example I%!&3000 or
osword_block%!1, then the two operands are added together to
form the effective address. The first operand, conventionally the
base address, must not be a constant. The second operand is the
offset, typically a constant offset into a MOS control block.
Whether unary or binary, the operator will either read or write to memory,
depending on whether it is evaluated as an expression, or assigned a
value. In the latter case the whole [<num-var>]!<numeric>
expression serves as an lvalue, or <num-var>.
For example, !osword_block% = target% sets the doubleword at
the address given by o7f_block%, to the value of
target%.
result% = osgbpb_block%!5 fetches the doubleword five to
eight bytes up from osgbpb_block%'s value, and returns it to
result%.
The binary operation is the highest priority operation in any expression.
For example, A%!PAGE+4 reads the word at
(A%+PAGE) and then adds 4 to it. A%!(PAGE+4)
reads the word at (A%+PAGE+4).
Doublewords in BBC BASIC are signed and little-endian: the byte at the
effective address holds the units place, the next one up holds the 256s
place, and so on. The whole doubleword is stored in two's-complement form
when the value is negative. If a fractional number is assigned to the
! operator, it is rounded toward zero.
Warnings
The BBC Microcomputer User Guide is peppered with warnings about the use
of ?, ! and $. They are not to be
used to access memory-mapped devices or the system's internal variables –
at least, not in published programs. The relevant addresses may change or
disappear on different machines and MOS versions, or the program may find
itself running on the other side of the Tube! The MOS offers a
comprehensive API to access system functions in a portable way.
Address space
The address space in which ! operates is the one BASIC
chooses to provide for !. Normally this is the address space
of the processor running BASIC. The BASIC program appears in this space,
between PAGE and TOP, but as mentioned
above, well-behaved programs must not alter it.
BAS128 for the B+ and Master puts the 6502
address space from 0 to &FFFF, and adds an extended space between &10000
and &1FFFF. This is made of the four slots of sideways RAM and
contains the user's program, variables and memory blocks. The MOS cannot
access this space (unless its own extended addressing system is enabled)
and machine code definitely cannot run in it although it can be assembled
there with OPTions 4 to 7.
-- beardo 04:43, 11 October 2007 (BST)
