Qemu HPET comparator_count is 2?

[zaptor]

According to https://wiki.osdev.org/HPET#HPET_-_timer_vs_comparators:

There're from 3 to 32 comparators, and the exact amount is indicated by comparator_count field in the above hpet structure.

However, when I print the value of comparator_count in the acpi HPET table, I'm seeing 2. I use the exact struct shown on that osdev page, namely:

Code: Select all

struct address_structure
{
    uint8_t address_space_id;    // 0 - system memory, 1 - system I/O
    uint8_t register_bit_width;
    uint8_t register_bit_offset;
    uint8_t reserved;
    uint64_t address;
} __attribute__((packed));
 
struct description_table_header
{
    char signature[4];    // 'HPET' in case of HPET table
    uint32_t length;
    uint8_t revision;
    uint8_t checksum;
    char oemid[6];
    uint64_t oem_tableid;
    uint32_t oem_revision;
    uint32_t creator_id;
    uint32_t creator_revision;
} __attribute__((packed));
 
struct hpet : public description_table_header
{
    uint8_t hardware_rev_id;
    uint8_t comparator_count:5;
    uint8_t counter_size:1;
    uint8_t reserved:1;
    uint8_t legacy_replacement:1;
    uint16_t pci_vendor_id;
    address_structure address;
    uint8_t hpet_number;
    uint16_t minimum_tick;
    uint8_t page_protection;
} __attribute__((packed));

When I print comparator_count, I'm seeing the value 2 which is not between 3 and 32. What's going on here?

[MollenOS]

It uses 5 bits to keep comparator count which technically the count can max be 31. This probably means that you need to add 1 to the actual count, as 0 = 1.... 31 = 32 to get the actual number of comparators.