?

The question mark, ?, is a BASIC operator to read or change bytes in memory. In this context it is pronounced 'query'. It is 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	3F (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 byte value at the effective address. In the second form, the value assigned to ? is written to the byte 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 byte at a time. A byte is a unit of storage that can hold a value between 0 and 255. By contrast the ! operator acts on a doubleword, or four consecutive 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 ?o7f_block%, then the single operand is used as the effective address.

If used as a binary operator, for example I%?&3000 or o7f_block%?6, 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, ?o7f_block% = drive% sets the byte at the address given by o7f_block%, to the value of drive%. Other BASICs would use POKE o7f_block%,drive%.

result% = o7f_block%?10 fetches the byte ten addresses up from o7f_block%'s value, and returns it to result%. This translates as result% = PEEK o7f_block%+10. (A more common idiom on the BBC Micro is result% = o7f_block%?(7+o7f_block%?5), which fetches another byte from the control block to calculate the offset of the result byte.)

The binary operation is the highest priority operation in any expression. For example, A%?PAGE+4 reads the byte at (A%+PAGE) and then adds 4 to it. A%?(PAGE+4) reads the byte at (A%+PAGE+4).

Bytes in BBC BASIC are unsigned, that is to say, between 0 and 255. If a number outside this range is assigned to the ? operator, it is rounded toward zero and the least significant byte is written to memory (in two's-complement form if necessary). For instance 1000.1 is written as 232, -254 becomes 2, and -253.5 is stored as 3.

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.

Bugs

? operators cannot be made LOCAL to, or be the formal parameters of, procedures or functions. For example, neither of the following lines are safe:

DEF FNfred(?block%)

LOCAL ?temp

In 6502 BASIC I to III, the BASIC stack can only hold integers and strings, not bytes. On entry to the procedure or function, the global value of the byte is saved on the stack as an integer. On exit, this integer is 'restored', overwriting four bytes instead of one. Furthermore it is restored to the wrong address with potentially harmful results.

The bug was fixed in 6502 BASIC III. ! operators are safe to use in this way from BASIC II onwards.

-- beardo 04:56, 11 October 2007 (BST)