ALCHP (space) or

ALCHP (space [,[jobID]]) (BTool only)

Location Toolkit II, THOR XVI, BTool

The function ALCHP allocates space bytes in the common heap and returns the start address of the memory set aside to be altered freely. This, unlike RESPR, works even if there is a task running in memory.

If ALCHP fails due to lack of available memory, then it will return 0 instead of breaking with error -3 (Out of Memory).

The BTool version of ALCHP allows an extended syntax. If space is followed by a comma ‘,’ then the allocated memory can only be removed with RECHP or CLCHP (unlike the other versions where this is done automatically with NEW and CLEAR). If the jobID is specified then not only will this be done, but the memory will also be linked to the Job identified by jobID.

Example 1

The following program loads two uncompressed screens from

disk into memory and shows them alternately:

100 adr=ALCHP(2*32768)
110 LBYTES flp1_Screen1_scr,adr
120 LBYTES flp1_Screen2_scr,adr+32768
130 REPeat Picture_Show
140   SCRBASE adr : REFRESH : PAUSE 150
150   SCRBASE adr+32768 : REFRESH : PAUSE 150
160 END REPeat Picture_Show

Example 2

This is an alternative to the LRESPR command (although see Note 2 below):

100 DEFine PROCedure LALCHP (mc_file$)
110  LOCal length,adress
120  length=FLEN(\mc_file$)
130  adress=ALCHP(length)
140  LBYTES mc_file$,adress
150  CALL adress


ALCHP reserves memory in 512 byte chunks.


Memory reserved by ALCHP is indirectly cleared by NEW, CLEAR, LOAD and LRUN (this does not apply to the Btool extended variant - see above).


Never run device drivers in the common heap - this memory can be easily cleared, causing a spectacular crash if a device driver was stored there. This is true for other machine code, too.


There is no checking on the parameter for ALCHP - accordingly negative values can be supplied. These are likely to lead to unexpected results and will probably crash the computer - for example, x=ALCHP(-100) crashes a JM ROM. On a Minerva ROM, values below -5 will return 0. On SMS although only values below -20 return 0, any attempt to reclaim the areas set aside with CLCHP or RECHP will crash the system.


Since ALCHP returns 0 if there is not enough memory, you should always check the value returned by ALCHP for this before writing to the address. Otherwise, it is possible that you will be over-writing the operating system… crash!


The reserved parts of memory can be given back to QDOS’ memory management by RECHP base_address or CLCHP.

RESPR, TTALL, ALLOCATION and especially GRAB and RESERVE work similar to ALCHP.

See DEL_DEFB concerning heap fragmentation.