A method of and an apparatus for searching a contents addressable memory

The present invention relates to a method of and an apparatus for searching a Contents Addressable Memory (hereafter abbreviated as the CAM) which can be accessed with its contents, that is, memorized data, as a search key

In the CAM, data memorized therein can be accessed with contents of the data themselves, while, in, ordinary memory devices, memorized data are simply to be accessed with their addresses. That is, when a bit sequence of a word length, whereof some bits may be masked, is given as a search key to the CAM in a search mode, the CAM outputs an address thereof by encoding an address line, namely, a word line of its memory cell array wherein the same or matching bit sequence is recorded.

Therefore, by memorizing communication log data in the CAM at addresses associated with their time stamps, for example, a time stamp of a specific communication log can be searched by accessing the CAM with a bit sequence corresponding to log data to be searched.

There may be more than one address line, however, wherein the same data are recorded in, the CAM array. Hence, there is usually provided a priority address encoder in the CAM, which outputs a searched address by encoding an address line selected with a certain priority, that is, a lowest address line or a highest address line among address lines of the CAM array which are made active by the search key.

Further, in some CAM devices, there is provided a priority restriction circuit for restricting effective address range of the address lines to be encoded with the priority by designating a restriction address, so that the priority encoder outputs an address of the lowest address line not lower than the restriction address, or an address of the highest address line not higher than the restriction address, among the address lines made active by the search key.

An example of a CAM provided with such a searching apparatus having the priority encoder and the priority restriction circuit is described by the present inventor in a Japanese patent application entitled "ATM Cell Transfer System" (hereafter called the first prior art), laid open as a Provisional Publication No. 139741/97. In the ATM cell transfer system, by associating addresses of the CAM with timings, address data of received ATM cells stored ii, a buffer memory are registered in lowest available addresses (corresponding to earliest timings) of the CAM searched with a search key for searching available addresses with the restriction addresses associated with timings (that is, after the timings) at which the received ATM cells are ideally to be transmitted, for enabling a prompt and simple re-timing of ATM cells to be transferred.

In another Japanese patent application laid open as a Provisional Publication No. 189979/93 (to be called the second prior art), a different type of the searching apparatus is disclosed, wherein all addresses of a CAM having the matching data are output sequentially one by one with a priority.

However, there are cases wherein a CAM having a limited memory space is desired to be used cyclically.

For example, there may be a case where specific log data earliest after 22 o'clock of the previous date are desired to be searched from communication log data daily recorded at every second in a CAM whereof 60 × 60 × 24 addresses are associated to every second of one day. In the case, designating a restriction address corresponding to 22 o'clock, the CAM is searched with a bit sequence corresponding to the specific log data. However, when there is found no matching log data after the restriction address until the highest address, that is, from 22 o'clock to 24 o'clock, the CAM should be searched once more, resetting the restriction address to the lowest address representing 0 o'clock of the present day, in the first prior art.

It is the same in the second prior art.

Therefore, a primary object of the present invention is to provide a method of and an apparatus for searching a CAM which enables to search an address of matching contents cyclically recorded in a memory array of the CAM with a priority at once, that is, a lowest address line indicating matching contents among whole address lines of the memory cell array when there is no address line indicating matching contents among address lines not lower than a designated restriction address, and a lowest address line indicating matching contents, when there is ally, among address lines not lower than the restriction address, or a highest address line indicating matching contents among whole address lines of the memory cell array when there is no address line indicating matching contents among address lines not higher than a restriction address, and a highest address line indicating matching contents, if there is any, among address lines not higher than the restriction address.

In order to achieve the object, the method of the invention of searching a CAM array having a plurality of first address lines whereof certain are made active when the CAM array is searched with a search key comprises steps of: <ul id="ul0001" list-style="none" compact="compact"><li>obtaining restricted search results by making address lines of the plurality of first address lines having addresses lower than a restriction address inactive;</li><li>selecting logic of third address lines from logic of the restricted search results when any of the restricted search results is active, and from logic of the plurality of the first address lines as it is, when none of the restricted search results is active; and</li><li>outputting a searched address by encoding an active address line having lowest address among the third address lines.</li></ul>

Therefore, an address of matching contents cyclically recorded in the CAM array can be searched with a priority at once, that is, a lowest address line indicating matching contents among whole address lines of the CAM array when there is no address line indicating matching contents among address lines not lower than a designated restriction address, and a lowest address line indicating matching contents, when there is any, among address lines not lower than the restriction address.

When a highest address is desired to be searched cyclically, the method of the invention comprises steps of: <ul id="ul0002" list-style="none" compact="compact"><li>obtaining restricted search results by making address lines of the plurality of first address lines having addresses higher than a restriction address inactive;</li><li>selecting logic of third address lines from logic of the restricted search results when any of the restricted search results is active, and from logic of the plurality of the first address lines as it is, when none of the restricted search results is active; and</li><li>outputting a searched address by encoding an active address line having, highest address among the third address lines.</li></ul>

Therefore, an address of matching contents cyclically recorded in the CAM array can be searched also with another priority at once, that is, a highest address line indicating matching contents among whole address lines of the CAM array when there is no address line indicating matching contents among address lines not higher than a designated restriction address, and a highest address line indicating matching contents, when there is any, among address lines not higher than the restriction address.

The foregoing, further objects, features, and advantages of this invention will become apparent from a consideration of the following description, the appended claims, and the accompanying drawings wherein the same numerals indicate the same or the corresponding parts.

In the drawings: <ul id="ul0003" list-style="none" compact="compact"><li>FIG. 1 is a block diagram of a CAM provided with a searching apparatus 1 according to an embodiment of the invention;</li><li>FIG. 2 is a block diagram illustrating an example of the selective restriction circuit 7 of FIG. 1; and</li><li>FIG. 3 is a block diagram illustrating another example of the selective restriction circuit 7 of FIG. 1.</li></ul>

Now, embodiments of the present invention will be described in connection with the drawings.

FIG. 1 is a block diagram of a CAM provided with a searching apparatus 1 according to an embodiment of the invention for obtaining a searched address by encoding one of first address lines 20 made active of a CAM array 2.

The CAM array 2 is provided with a search key interface 3 where a search key is input and a mask data interface 4 where mask data are input, and makes active each of the first address lines 20 connected to memory cells storing contents whereof logic of bits unmasked by the mask data is matching with logic of corresponding bits of the search key.

The searching apparatus 1 comprises; <ul id="ul0004" list-style="none" compact="compact"><li>an address decoder 6 for making one of second address lines 60 active by decoding a restriction address supplied thereto, the second address lines 60 having the same number with the first address lines 20,</li><li>a selective restriction circuit 7 for restricting effective range of the first address lines 20 selectively, such as will be described afterwards, according to logic of the first address lines 20 and logic of the second address lines 60, and making active some of third address lines 70 according to logic of effective range of the first address lines 20, and</li><li>an priority address encoder 13 for outputting a searched address by encoding an active one of the third address lines 70 with a priority.</li></ul>

As to the address decoder 6, any appropriate conventional address decoder, such as a row address decoder of an ordinary memory cell array for making one of word lines active by decoding a row address, may be applied, and detailed descriptions are omitted, here.

As to the priority address encoder 13, any appropriate conventional priority address encoder may be applied, and detailed descriptions are also omitted. When a lowest address is desired to be searched cyclically, an priority address encoder for encoding a lowest active address line is applied to the priority address encoder 13, and an priority address encoder for encoding a highest active address line is applied to the priority address encoder 13 when a highest address is desired to be searched cyclically.

In the following paragraphs, details of the selective restriction circuit 7 will be described.

FIG. 2 is a block diagram illustrating an example of the selective restriction circuit 7 which is applied to the searching apparatus 1 wherein a lowest address is desired to be searched cyclically, by way of example, used in combination with a priority address encoder 13 for encoding a lowest active address line.

Referring to FIG. 2, the selective restriction circuit 7 comprises; <ul id="ul0005" list-style="none" compact="compact"><li>a cascade connection of priority restriction circuits 8_<i>i</i> (<i>i</i> = 1 to <i>n</i>), each corresponding to each of a plurality (<i>n</i>) of the first address lines 20_<i>i</i>, for obtaining restricted search results 80_<i>i</i> according to logic of the second address lines 60_<i>i</i>,</li><li>a check circuit 9 for checking logic of the restricted search results 80_<i>i</i>, and</li><li>a cascade connection of selection circuits 10_<i>i</i> for selecting logic to be output to the third address lines 70_<i>i</i> from logic of either of the first address lines 20_<i>i</i> or the restricted search results 80_<i>i</i> according to output of the check circuit 9.</li></ul>

The first, the second and the third address lines 20_<i>i</i>, 60_<i>i</i> and 70_<i>i</i> correspond to those 20, 60 and 70 of FIG. 1.

Each of the priority restriction circuits, a priority restriction circuit 8_<i>i</i>, for example, has an OR gate 81 (except for first priority restriction circuit 8₁) for obtaining OR logic of <i>i</i>-th second address line 60_<i>i</i> and output of the OR gate 81 of the lower priority restriction circuit 8_<i>i</i>_-1, and an AND gate 82 for obtaining AND logic of <i>i</i>-th first address line 20_<i>i</i> and output of the OR gate 81 (or logic of a first 60₁ of the second address lines in the first priority restriction circuit 8₁).

The check circuit 9 has an multi-input NOR gate 91 for obtaining NOR logic of all of the restricted search results 80_<i>i</i>.

Each of the selection circuits, a selection circuit 10_<i>i</i>, for example, has an AND gate 101 for obtaining AND logic of <i>i</i>-th first address line 20_<i>i</i> and output of the NOR gate 91, and an OR gate 102 for obtaining OR logic of <i>i</i>-th search result 80_<i>i</i> and output of the AND gate 101.

Now, operation of the selective restriction circuit is described referring to FIGs. 1 and 2.

A restriction address <i>i</i>, for example, supplied to the address decoder 6 is decoded and corresponding one of the second address lines 60, an <i>i</i>-th second address line 60_<i>i</i>, becomes active. Hence, output of the OR gate 81 becomes active in the priority restriction circuits 8_<i>i</i> to 8_<i>n</i>, while it remains inactive in the priority restriction circuits 8₁ to 8_<i>i</i>_-1. Therefore, the output of the AND gate 82 remains inactive in the priority restriction circuits 8₁ to 8_<i>i</i>_-1 regardless of logic of the first address line 20₁ to 20_<i>i</i>_-1, while it becomes the same with first address line 20<i>i</i> in the priority restriction circuits 8_<i>i</i> to 8_<i>n</i>.

Thus, the restricted search results 80₁ to 80_<i>n</i> are obtained from the AND gate 82 of the priority restriction circuits 8₁ to 8_<i>n</i> by masking search results of the first address lines 20₁ to 20_<i>i</i>_-1 whereof addresses are lower than the search restriction address <i>i</i>.

When there is any which is active among the restricted search results 80₁ to 80_<i>n</i>, output of the NOR gate 91 becomes inactive. Hence, output of the AND gate 101 of every selection circuit 10₁ to 10_<i>n</i> remains inactive regardless of logic of the first address lines 20₁ to 20_<i>n</i>. Therefore, logic of the restricted search results 80₁ to 80_<i>n</i> are output to the third address lines 70₁ to 70_<i>n</i>, respectively, as they are.

On the other hand, when every of the restricted search results 80₁ to 80_<i>n</i> is inactive, that is, no search result is found from the restriction address to the maximum address, output of the NOR gate 91 becomes active. Hence, output of the AND gate 101 of every selection circuit 10₁ to 10_<i>n</i> becomes the same with logic of the first address lines 20₁ to 20_<i>n</i>. Therefore, logic of the first address lines 20₁ to 20_<i>n</i> is output to the third address lines 70₁ to 70_<i>n</i>, respectively, as they are.

The third address lines 70 (70₁ to 70_<i>n</i>) are encoded by the priority address encoder 13 and an address of the lowest active one of the third address lines 70 is output as the searched address.

Thus, the searching apparatus 1 of the embodiment enables to search an address of matching contents cyclically recorded in the CAM array 2 with a priority at once, that is, a lowest address line indicating matching contents among whole address lines of the CAM array 2 when there is no address line indicating matching contents among address lines not lower than a designated restriction address, and a lowest address line indicating matching contents, when there is any, among address lines not lower than the restriction address.

FIG. 3 is a block diagram illustrating another example of the selective restriction circuit 7 of FIG. 1 which is applied to the searching apparatus 1 wherein a highest address is desired to be searched cyclically, in combination with a priority address encoder 13 which encodes a highest active one of the third address lines 70.

The selective restriction circuit 7 of FIG. 3 has a similar configuration to the selective restriction circuit 7 of FIG. 2 except for comprising a cascade connection of priority restriction circuits 11_<i>i</i> (<i>i</i> = 1, to <i>n</i>), in place of the priority restriction circuits 8_<i>i</i> of FIG. 2.

Each of the priority restriction circuits, a priority restriction circuit 11_<i>i</i>, for example, has an OR gate 83 (except for <i>n</i>-th priority restriction circuit 11_<i>n</i>) for obtaining OR logic of <i>i</i>-th second address line 60_<i>i</i> and output of the OR gate 83 of the upper priority restriction circuit 11_<i>i</i>₊₁, and an AND gate 82 for obtaining AND logic of <i>i</i>-th first address line 20_<i>i</i> and output of the OR gate 83 (or logic of the <i>n</i>-th second address line 60_<i>n</i> itself in the <i>n</i>-th priority restriction circuit 11<i>n</i>).

Therefore, by designating a restriction address <i>i</i>, the restricted search results 80₁ to 80_<i>n</i> are obtained from the AND gate 82 of the priority restriction circuits 11₁ to 11_<i>n</i> by masking search results of the first address lines 20_<i>i</i>₊₁ to 20_<i>n</i> whereof addresses are higher than the search restriction address <i>i</i>, in the similar way with the priority restriction circuit 8₁ to 8_<i>n</i> of FIG. 2.

The check circuit 9 checks logic of the restricted search results 80₁ to 80_<i>n</i>, and the selection circuits 10₁ to 10_<i>n</i> selects logic to be output to the third address lines 70 from logic of either of the first address lines 20 or the restricted search results 80₁ to 80_<i>n</i> according to output of the check circuit 9, in the same way with the check circuit 9 and the selection circuits 10₁ to 10_<i>n</i> of FIG. 2.

The third address lines 70 are encoded by the priority address encoder 13 and an address of the highest active one of the third address lines 70 is output as the searched address.

Thus, the searching apparatus 1 of this embodiment enables to search an address of matching contents cyclically recorded in the CAM array 2 with a priority at once, that is, a highest address line indicating matching contents among whole address lines of the CAM array 2 when there is no address line indicating matching contents among address lines not higher than a designated restriction address, and a highest address line indicating matching contents, when there is any, among address lines not higher than the restriction address.