ExpressionsExpressions are used to directly represent numbers or strings. Since arithmetic operations are not included in the current assembly language, expressions are nothing but "literals".
As Heritage/1 is a strict 16-bits machine (memory is organized in 16-bits words), numerical expressions are always 16-bits integers (singed or unsigned) and values greater than 65,535 will generate overflow errors at assembling-time. Bytes (8-bits) are NOT represented within assembly code in any way. NOTE: A notable exception to this is the instruction INT which takes an 8-bits vector number as argument in assembly code (this byte is actually embedded within the op. code); in this case the argument is written as a normal 16-bits word although the assembler will report overflow error if it is greater than 0x00ff.
The following numerical expressions are valid:
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0x03ff Hexadecimal
0b010101 Binary
44 Decimal
-44 Decimal negative
65000 Decimal. Valid but it may represent a negative binary (two's complement)
The following expressions are illegal:
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-0x03ff Only decimals can be explicitly signed.
68456 Overflow.
64,000 Commas as delimiters in numbers are not accepted.
A string is a null-terminated sequence of ASCII characters occupying a 16-bits word each (the assembler fills the MSB with zeros). The null termination consists of a word with all bits cleared.
Strings are written in assembly code starting with dollar sign ($) as in the following example:
:ERROR_MSGS
#data $Drive not on-line
#data $Volume not mounted on drive
A words about signed and unsigned integers
The Heritage/1 ALU makes use of two's complement arithmetic for adding and subtracting 16-bits numbers. However, this does not limit the programmer to the use of signed integers in assembly code, as signed or unsigned is mostly a matter of interpretation.
For instance, using the instruction ADD for adding 0xf000 to 0x0001 will result in 0xf001 which can be interpreted either as positive 61441 or negative or -4095. Moreover, when building more powerful arithmetic by software (such as BCD or Floating Point) the programmer is responsibly for defining data types and to make the correct representation and interpretation of the arithmetic sign.
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