`ti_Read` and `ti_Write` assumes `ti._smulu` sets UHL to zero

[ZERICO2005]

; size_t ti_Write(const void *data, size_t size, size_t count, uint8_t handle);
ti_Write:
; writes a chunk of data into a slot handle
; args:
;  sp + 3 : pointer to data to write
;  sp + 6 : size of chunks in bytes
;  sp + 9 : number of chunks
;  sp + 12 : slot index
; return:
;  hl = number of chunks written if success
    ld    iy, 0
    add    iy, sp
    ld    c, (iy + 12)
    call    util_is_slot_open
    jr    nz, .ret0
    call    util_is_in_ram
    jr    z, .ret0
    ld    bc, (iy + 6) ; size
    ld    hl, (iy + 9) ; count
    call    ti._smulu ; size * count
    add    hl, de
    xor    a, a
    sbc    hl, de
    ret    z
    ld    (.smc_copy_size), hl
    ; ...

This code assumes ti._smulu sets UHL to zero. While I assume 0 * 0 will set UHL to zero, this may not be the case for non-zero multipliers.

If UHL is non-zero, that implies that .smc_copy_size will copy more than 65535 bytes.