diff --git a/client/app_config.cpp b/client/app_config.cpp
index 143c51c9b6..c763720719 100644
--- a/client/app_config.cpp
+++ b/client/app_config.cpp
@@ -27,13 +27,49 @@
 
 bool have_max_concurrent = false;
 
+int APP_CONFIG::parse_gpu_versions(XML_PARSER& xp, PROJECT* p) {
+    double x;
+    while (!xp.get_tag()) {
+        if (xp.match_tag("/gpu_versions")) return 0;
+        else if (xp.parse_double("gpu_usage", x)) {
+            if (x <= 0) {
+                msg_printf(p, MSG_USER_ALERT,
+                    "gpu_usage must be positive in app_config.xml"
+                );
+            } else {
+                gpu_gpu_usage = x;
+            }
+            continue;
+        }
+        else if (xp.parse_double("cpu_usage", x)) {
+            if (x < 0) {
+                msg_printf(p, MSG_USER_ALERT,
+                    "cpu_usage must be non-negative in app_config.xml"
+                );
+            } else {
+                gpu_cpu_usage = x;
+            }
+            continue;
+        }
+        if (log_flags.unparsed_xml) {
+            msg_printf(p, MSG_INFO,
+                "Unparsed line in app_config.xml: %s",
+                xp.parsed_tag
+            );
+        }
+    }
+    msg_printf_notice(p, false, NULL,
+        "missing </gpu_versions> in app_config.xml"
+    );
+    return ERR_XML_PARSE;
+}
+
 // In these parsing functions, if there's an error you must
 // - generate a notice containing the string "app_config.xml"
 // - return an error code
 //
 int APP_CONFIG::parse(XML_PARSER& xp, PROJECT* p) {
     memset(this, 0, sizeof(APP_CONFIG));
-    double x;
 
     while (!xp.get_tag()) {
         if (xp.match_tag("/app")) return 0;
@@ -43,29 +79,8 @@ int APP_CONFIG::parse(XML_PARSER& xp, PROJECT* p) {
             continue;
         }
         if (xp.match_tag("gpu_versions")) {
-            while (!xp.get_tag()) {
-                if (xp.match_tag("/gpu_versions")) break;
-                if (xp.parse_double("gpu_usage", x)) {
-                    if (x <= 0) {
-                        msg_printf(p, MSG_USER_ALERT,
-                            "gpu_usage must be positive in app_config.xml"
-                        );
-                    } else {
-                        gpu_gpu_usage = x;
-                    }
-                    continue;
-                }
-                if (xp.parse_double("cpu_usage", x)) {
-                    if (x < 0) {
-                        msg_printf(p, MSG_USER_ALERT,
-                            "cpu_usage must be non-negative in app_config.xml"
-                        );
-                    } else {
-                        gpu_cpu_usage = x;
-                    }
-                    continue;
-                }
-            }
+            int retval = parse_gpu_versions(xp, p);
+            if (retval) return retval;
             continue;
         }
         if (xp.parse_bool("fraction_done_exact", fraction_done_exact)) {
diff --git a/client/app_config.h b/client/app_config.h
index 1dc88ab6f7..0bb198f3dd 100644
--- a/client/app_config.h
+++ b/client/app_config.h
@@ -35,6 +35,7 @@ struct APP_CONFIG {
     bool fraction_done_exact;
 
     int parse(XML_PARSER&, PROJECT*);
+    int parse_gpu_versions(XML_PARSER&, PROJECT*);
 };
 
 struct APP_VERSION_CONFIG {
diff --git a/client/client_state.cpp b/client/client_state.cpp
index 16779b8f0f..bf19631a01 100644
--- a/client/client_state.cpp
+++ b/client/client_state.cpp
@@ -423,6 +423,15 @@ int CLIENT_STATE::init() {
             coprocs.coprocs[j].type,
             coprocs.coprocs[j].count
         );
+        if (coprocs.coprocs[j].count > MAX_COPROC_INSTANCES) {
+            msg_printf(NULL, MSG_USER_ALERT,
+                "%d instances of %s specified in cc_config.xml; max is %d",
+                coprocs.coprocs[j].count,
+                coprocs.coprocs[j].type,
+                MAX_COPROC_INSTANCES
+            );
+            coprocs.coprocs[j].count = MAX_COPROC_INSTANCES;
+        }
     }
     if (!cc_config.no_gpus
 #ifdef _WIN32
diff --git a/client/client_types.h b/client/client_types.h
index c7d753f02d..b299a7fd3d 100644
--- a/client/client_types.h
+++ b/client/client_types.h
@@ -258,7 +258,7 @@ struct APP {
         // Can also specify in client_state.xml (for client emulator)
     int n_concurrent;
         // temp during job scheduling, to enforce max_concurrent
-    int non_excluded_instances[MAX_RSC];
+    COPROC_INSTANCE_BITMAP non_excluded_instances[MAX_RSC];
         // for each resource type, bitmap of the non-excluded instances
 #ifdef SIM
     double latency_bound;
diff --git a/client/gpu_detect.cpp b/client/gpu_detect.cpp
index 2d73b9c675..76628d3306 100644
--- a/client/gpu_detect.cpp
+++ b/client/gpu_detect.cpp
@@ -304,9 +304,11 @@ void COPROCS::correlate_gpus(
     cpu_opencls.clear();
 }
 
-// This is called from CLIENT_STATE::init() after adding NVIDIA, ATI and Intel GPUs
+// This is called from CLIENT_STATE::init()
+// after adding NVIDIA, ATI and Intel GPUs
 // If we don't care about the order of GPUs in COPROCS::coprocs[], 
 // this code could be included at the end of COPROCS::correlate_gpus().
+//
 int COPROCS::add_other_coproc_types() {
     int retval = 0;
     
@@ -318,8 +320,10 @@ int COPROCS::add_other_coproc_types() {
         }
         
         COPROC c;
-        // For other device types other than NVIDIA, ATI or Intel GPU coprocessor.
-        // we put each instance into a separate other_opencls element, so count=1.
+        // For device types other than NVIDIA, ATI or Intel GPU.
+        // we put each instance into a separate other_opencls element,
+        // so count=1.
+        //
         c.count = 1;
         c.opencl_device_count = 1;
         c.opencl_prop = other_opencls[i];
diff --git a/client/log_flags.cpp b/client/log_flags.cpp
index 223176ea2b..5196d0c53e 100644
--- a/client/log_flags.cpp
+++ b/client/log_flags.cpp
@@ -592,7 +592,9 @@ void process_gpu_exclusions() {
         APP* app = gstate.apps[i];
         for (int k=1; k<coprocs.n_rsc; k++) {
             COPROC& cp = coprocs.coprocs[k];
-            app->non_excluded_instances[k] = (1<<cp.count)-1;  // all 1's
+            for (int h=0; h<cp.count; h++) {
+                app->non_excluded_instances[k] |= ((COPROC_INSTANCE_BITMAP)1)<<h;
+            }
         }
     }
 
@@ -600,18 +602,23 @@ void process_gpu_exclusions() {
         p = gstate.projects[i];
         for (int k=1; k<coprocs.n_rsc; k++) {
             COPROC& cp = coprocs.coprocs[k];
-            int all_instances = (1<<cp.count)-1;  // bitmap of 1 for all inst
+            COPROC_INSTANCE_BITMAP all_instances = 0;
+            // bitmap of 1 for all instances
+            //
+            for (int h=0; h<cp.count; h++) {
+                all_instances |= ((COPROC_INSTANCE_BITMAP)1)<<h;
+            }
             for (j=0; j<cc_config.exclude_gpus.size(); j++) {
                 EXCLUDE_GPU& eg = cc_config.exclude_gpus[j];
                 if (!eg.type.empty() && (eg.type != cp.type)) continue;
                 if (strcmp(eg.url.c_str(), p->master_url)) continue;
-                int mask;
+                COPROC_INSTANCE_BITMAP mask;
                 if (eg.device_num >= 0) {
                     int index = cp.device_num_index(eg.device_num);
                     // exclusion may refer to nonexistent GPU
                     //
                     if (index < 0) continue;
-                    mask = 1<<index;
+                    mask = ((COPROC_INSTANCE_BITMAP)1)<<index;
                 } else {
                     mask = all_instances;
                 }
@@ -652,7 +659,7 @@ void process_gpu_exclusions() {
             //
             p->rsc_pwf[k].ncoprocs_excluded = 0;
             for (int b=0; b<cp.count; b++) {
-                int mask = 1<<b;
+                COPROC_INSTANCE_BITMAP mask = ((COPROC_INSTANCE_BITMAP)1)<<b;
                 for (a=0; a<gstate.apps.size(); a++) {
                     APP* app = gstate.apps[a];
                     if (app->project != p) continue;
diff --git a/client/rr_sim.cpp b/client/rr_sim.cpp
index 33a3fffd3f..a8b7e660b5 100644
--- a/client/rr_sim.cpp
+++ b/client/rr_sim.cpp
@@ -64,12 +64,14 @@ inline void rsc_string(RESULT* rp, char* buf) {
 
 // set "nused" bits of the source bitmap in the dest bitmap
 //
-static inline void set_bits(int src, double nused, int& dst) {
+static inline void set_bits(
+    COPROC_INSTANCE_BITMAP src, double nused, COPROC_INSTANCE_BITMAP& dst
+) {
     // if all bits are already set, we're done
     //
     if ((src&dst) == src) return;
-    int bit = 1;
-    for (int i=0; i<32; i++) {
+    COPROC_INSTANCE_BITMAP bit = 1;
+    for (int i=0; i<MAX_COPROC_INSTANCES; i++) {
         if (nused <= 0) break;
         if (bit & src) {
             dst |= bit;
@@ -524,11 +526,14 @@ void RR_SIM::simulate() {
         RSC_WORK_FETCH& rwf = rsc_work_fetch[i];
         if (!rwf.has_exclusions) continue;
         COPROC& cp = coprocs.coprocs[i];
-        int mask = (1<<cp.count)-1;
+        COPROC_INSTANCE_BITMAP mask = 0;
+        for (int j=0; j<cp.count; j++) {
+            mask |= ((COPROC_INSTANCE_BITMAP)1)<<j;
+        }
         rwf.sim_excluded_instances = ~(rwf.sim_used_instances) & mask;
         if (log_flags.rrsim_detail) {
             msg_printf(0, MSG_INFO,
-                "[rrsim_detail] rsc %d: sim_used_inst %d mask %d sim_excluded_instances %d",
+                "[rrsim_detail] rsc %d: sim_used_inst %lld mask %lld sim_excluded_instances %lld",
                 i, rwf.sim_used_instances, mask, rwf.sim_excluded_instances
             );
         }
diff --git a/client/work_fetch.h b/client/work_fetch.h
index 182820de2c..6c0ece63dc 100644
--- a/client/work_fetch.h
+++ b/client/work_fetch.h
@@ -61,6 +61,9 @@ struct RSC_WORK_FETCH;
 struct SCHEDULER_REPLY;
 struct APP_VERSION;
 
+typedef long long COPROC_INSTANCE_BITMAP;
+    // should be at least MAX_COPROC_INSTANCES (64) bits
+
 // state per (resource, project) pair
 //
 struct RSC_PROJECT_WORK_FETCH {
@@ -88,7 +91,7 @@ struct RSC_PROJECT_WORK_FETCH {
     double nused_total;     // sum of instances over all runnable jobs
     int ncoprocs_excluded;
         // number of excluded instances
-    int non_excluded_instances;
+    COPROC_INSTANCE_BITMAP non_excluded_instances;
         // bitmap of non-excluded instances
         // (i.e. instances this project's jobs can run on)
     int deadlines_missed;
@@ -200,10 +203,10 @@ struct RSC_WORK_FETCH {
         // seconds of idle instances between now and now+work_buf_total()
     double nidle_now;
     double sim_nused;
-    int sim_used_instances;
+    COPROC_INSTANCE_BITMAP sim_used_instances;
         // bitmap of instances used in simulation,
         // taking into account GPU exclusions
-    int sim_excluded_instances;
+    COPROC_INSTANCE_BITMAP sim_excluded_instances;
         // bitmap of instances not used (i.e. starved because of exclusion)
     double total_fetchable_share;
         // total RS of projects from which we could fetch jobs for this device