QDOS+ Basic Memory Areas

There are 12 distinct areas:

LOW ADDRESS
job header	-- Same layout as any other JB area. $68 bytes. SV_BASIC points to the base of this area.
SuperBASIC work area	-- Interpreter work variables. fixed length: includes pointers to other areas, which may be moved around. During interpretation, A6 points to the base of this area: the BV values are the offsets: the whole area can be moved (hence A6 is changed to new base address). See below.
name table	-- For access to SuperBasic variables and parameters. A table of 8-byte entries: see below.
namelist	-- For access to SuperBasic variables and parameters. Each entry is a BYTE character count, then the characters of the name [ALL OTHER STRINGS USE*WORD* -LENGTH COUNTS].
variable values area	-- For access to SuperBasic variables and parameters. A heap in which the values of the variables are cstored: see below for the formats of the entries.
channel table	-- For SuperBASIC channels. See below.
arithmetic (RI) stack	-- Convenient for reserving space in added routines: and for passing parameters from SuperBASIC to routines. See above.
token list	-- Not described in TechRefMan
line number table	-- Not described in TechRefMan
program file	-- Not described in TechRefMan
return list	-- Not described in TechRefMan
buffer	-- Not described in TechRefMan
HIGH ADDRESS

1 SuperBASIC work area

2 NAME TABLE ENTRIES

• A positive name pointer is to an entry in the name list: -1 is a nameless item: other -ve name pointers are to another entry in the name table of which this entry is a copy.
• A positive value pointer is to an entry in the variable values area: -1 is for an undefined value.
• Because the areas are movable, the pointers are offsets: for the RI stack the offsets are from the high-address base; for other areas it is from the low-address base.

[1] Byte 0 has bitwise format h sss tttt where

h # before the parameter 0 no/not param 1 yes (Only tttt is shown in the main table)

sss separator after the parameter 0 none/not param 1 comma 2 semi-colon 3 backslash 4 exclamation mark 5 'TO' keyword

tttt type of entry 0 null 1 string 2 f.p. 3 integer

[2] Not normally reccomended for use by added routines.

3 VARIABLE VALUES AREA

A heap in which the values are stored. The item entries are:

integer	-- 16-bit two's complement word.
f.p.	-- 2-byte exponent, then 4-byte mantissa. Ms 4 bits of exponent are 0, LS 12 bits are offset -$800. Mantissa is 2's complement with bit 31 -1, bit 30 +1/2, normalized so that bit31 /= bit30 (unless the whole number is exactly zero, in which case all 6 bytes are 0). So value is * mantissa*2^(exponent-$800) if the mantissa is taken as fraction or * mantissa*2^(exponent-$81F) if the mantissa is taken as .L integer.
string	-- 2 byte character count, then that number of characters, then (if the count was odd) an ignored byte to make the whole an even # of bytes
array	-- header of format * .L offset from base of variable values area to the array values * .W number of dimensions * .W max index of the dimension ** * .W index multiplier for the dimension ** [** The last pair of items are repeated once for each dimension.] STRING ARRAY: is an array of characters, except zeroth element of the final dimension is .W containing the string length; Final dimension defines the maximum string length, +1 if odd. SUBSTRING: result of a slicing: an array of characters, except that the base of indexing is one rather than zero.

4 THE BASIC CHANNEL TABLE

A channel number #n indexes the SuperBASIC channel table. Each entry in the table is of length CH_LENCH ($28) bytes. The base of the table is at BV(CHBAS(A6) and the top at BV_CHP(A6). So the base for entry #n is at ( BV_CHBAS(A6) + CH_LENCH*n ) (A6)

Each entry is of format:

$00.L	The channel ID
$04.F	Current graphics cursor x
$0A.F	Current graphics cursor y
$10.F	Turtle angle (degrees)
$16.B	Pen status
$20.W	Character position on line for PRINT and INPUT
$22.W	WIDTH of page

The table has as many entries as the number of the current highest-numbered channel: an un-opened channel either has a number greater than that, OR has a negative channel ID.

Last updated 4th January 1999. Back to my QL home page Disclaimer