!

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> = [<numeric>]!<numeric> [<numeric>]!<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 [<numeric>]!<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)