Dynamic memory allocation in an RSX

[ralferoo]

Just wondering if anyone knows how to get a block of free memory temporarily when an RSX is called. Obviously, I don't want to permanently reserve memory for a temporary buffer, but there's not enough space on the stack for the data I need to store.Basically, what I need is the last byte used by the BASIC runtime and the value of HIMEM. According to http://cpcrulez.fr/firmware.txt these seem useful:

     &AE5E ³ &AE7B ³ 2    ³ HIMEM (set by MEMORY)
     &AE6C ³ &AE89 ³ 2    ³ address of start of free space (wherenext Array entry is placed)

But obviously, this relies on also working out what version of BASIC is running and then looking in the correct places.Also, some initial analysis shows that the strings are stored just below HIMEM (pointed to at &AE5E), so that can't be considered the last free byte. The area pointed to by &AE6C does seem free, but I'm not sure if that's the correct thing to do. Obviously, I'm still not sure how much memory I can allocate either...So, I thought I'd ask you lot if this problem has already been solved, because I'm sure it must have been! Can anyone point to any source, preferably one that auto-detects which version of BASIC is being used.

EDIT: Maybe this is the variable I need to look at?
    &B071 ³ &B08D ³ 2    ³ address of end of free space (the byte before the Strings area)
    &B073 ³ &B08F ³ 2    ³ address of end of Strings area (=HIMEM)

[ralferoo]

If anyone's interested, this is what I've come up with. I'd appreciate comments though if people think it won't work:

Code Select Expand


do_mem_info:
    ld c,-1
try_next_rom:            ; increment count and test next rom
    inc c

    ld a,#10
    cp c
    ret z

    call #b915        ; kl probe rom
    cp #80            ; foreground and BASIC
    jr nz,try_next_rom

    ld a,l            ; check version number is 1
    dec l
    jr nz,try_next_rom

    ld a,h            ; mark (subversion) is 0=464, 1=664, 2=6128
    or a
    push af            ; zero flag set if 464

    ld hl,mem_info1_msg
    call print

    pop af
    push af

    ld hl,#ae89        ; holds lowmem on 464
    jr z, lowmem_464
    ld hl,#ae6c        ; holds lowmem on 664/6128
lowmem_464:
    call outhex_hlhl
    ex de,hl

    ld hl,mem_info2_msg
    call print

    pop af

    ld hl,#b08d        ; holds himem-strings on 464
    jr z, himem_464
    ld hl,#b071        ; holds himem-strings on 664/6128
himem_464:
    call outhex_hlhl

    and a
    sbc hl,de
    inc hl
    push hl

    ld hl,mem_info3_msg
    call print

    pop hl
    call outhex_hl

    ld hl,mem_info4_msg
    jp print

mem_info1_msg: defb "Free memory &",0
mem_info2_msg: defb " - &",0
mem_info3_msg: defb " (&",0
mem_info4_msg: defb " bytes)",13,10,0


One thing I'm suspicious of is if an RSX is launched from a non-BASIC foreground ROM...