ISCA is the top forum for architecture and has been so since 1975. ISCA papers are 10-12 pages in length with detailed results, and go through around 5-6 double-blind reviews by the top experts on the topic. The acceptance rate is 15-20%, decided by a NSF-panel-style, 20-person program committee. ISCA takes only 30-35 papers a year (there are no short papers, no posters).
Micro is more specialized than ISCA in that Micro targets microprocessor architecture (microarchitecture) rather than all architecture which includes multiprocessors and large computer systems. Micro has been the top forum for microarchitecture since 1970. Micro is similar to ISCA in all other aspects (10-12 pages in length with detailed results, and go through around 5-6 double-blind reviews by the top experts on the topic) except that the acceptance rate is about 20-25% decided by a NSF-panel-style, 20-person program commitee. Micro also takes only about 30-35 papers a year.
HPCA has the same scope as ISCA, has been around since 1994, and is similar to ISCA in all other aspects (10-12 pages in length with detailed results, and go through around 5-6 double-blind reviews by the top experts on the topic) except that the acceptance rate is about 20-25% decided by a NSF-panel-style, 20-person program commitee. HPCA also takes only about 30-35 papers a year and sometimes the submission is not blind.
ASPLOS is the top forum for ideas combining architecture, programming languages, and operating systems. While ISCA focuses on architecture, ASPLOS targets issues and ideas overlapping architectures, programming languages, and operating systems. ASPLOS is held once every two years, and is similar to ISCA in all other aspects (10-12 pages in length with detailed results, and go through around 5-6 double-blind reviews by the top experts on the topic) except that the acceptance rate is about 15-20% decided by a NSF-panel-style, 20-person program commitee. ASPLOS takes only about 25-30 papers every two years.