Index: src/pidp8i/gpio-common.c.in
==================================================================
--- src/pidp8i/gpio-common.c.in
+++ src/pidp8i/gpio-common.c.in
@@ -1,9 +1,9 @@
 /*
  * gpio-common.c: functions common to both gpio.c and gpio-nls.c
  *
- * Copyright © 2015 Oscar Vermeulen, © 2016-2019 by Warren Young
+ * Copyright © 2015, 2019 Oscar Vermeulen, © 2016-2019 by Warren Young
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
  * to deal in the Software without restriction, including without limitation
  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
@@ -580,24 +580,74 @@
         INP_GPIO (ledrows[row]);
     }
 }
 
 
-//// init_pidp8i_gpio //////////////////////////////////////////////////
-// Initialize the GPIO pins to the initial states required by
-// gpio_thread().   It's a separate exported function so that scanswitch
-// can also use it.
-
-void init_pidp8i_gpio (void)
-{
-    // Set GPIO pins to their starting state
-    turn_on_pidp8i_leds ();
-    for (size_t i = 0; i < NROWS; i++) {       // Define rows as input
-        INP_GPIO (rows[i]);
-    }
-
-    // BCM2835 ARM Peripherals PDF p 101 & elinux.org/RPi_Low-level_peripherals#Internal_Pull-Ups_.26_Pull-Downs
+//// init_bcm2711_gpio /////////////////////////////////////////////////
+// Called from init_pidp8i_gpio() when it sees that it's running on a Pi
+// 4 which uses the BCM 2711 GPIO peripheral, which behaves differently
+// than GPIO was done in all prior Pi models.  This code is based on
+// https://github.com/RPi-Distro/raspi-gpio/blob/master/raspi-gpio.c
+//
+// Note that there is no "serial mod" handling here: no one has come up
+// with such a hardware hack for the Pi 4 yet, and no one has verified
+// that you can do one of the existing ones atop the Pi 4.
+
+static void init_bcm2711_gpio (void)
+{
+    int gpiox, pullreg, pullshift;
+    unsigned int pullbits, pull;
+
+    // Configure "columns" GPIO pins
+    for (size_t i = 0; i < 12; ++i) {
+        gpiox = cols[i];
+        pullreg = GPPUPPDN0 + (gpiox>>4);
+        pullshift = (gpiox & 0xf) << 1;
+        pull = 1;
+
+        pullbits = *(gpio.addr + pullreg);
+        pullbits &= ~(3 << pullshift);
+        pullbits |= (pull << pullshift);
+        *(gpio.addr + pullreg) = pullbits;
+    }
+
+    // Configure "rows" GPIO pins
+    for (size_t i = 0; i < 3; ++i) {
+        gpiox = rows[i];
+        pullreg = GPPUPPDN0 + (gpiox>>4);
+        pullshift = (gpiox & 0xf) << 1;
+        pull = 0;
+
+        pullbits = *(gpio.addr + pullreg);
+        pullbits &= ~(3 << pullshift);
+        pullbits |= (pull << pullshift);
+        *(gpio.addr + pullreg) = pullbits;
+    }
+
+    // Configure "ledrows" GPIO pins
+    for (size_t i = 0; i < 6; ++i) {
+        gpiox = ledrows[i];
+        pullreg = GPPUPPDN0 + (gpiox>>4);
+        pullshift = (gpiox & 0xf) << 1;
+        pull = 0;
+
+        pullbits = *(gpio.addr + pullreg);
+        pullbits &= ~(3 << pullshift);
+        pullbits |= (pull << pullshift);
+        *(gpio.addr + pullreg) = pullbits;
+    }
+}
+
+
+//// init_bcm2835_gpio /////////////////////////////////////////////////
+// Called from init_pidp8i_gpio() when it sees that it's running on a Pi
+// 3 or older.  This is the "classic" Pi GPIO mode.  See page 101 in the
+// peripheral's datasheet and // http://elinux.org/RPi_Low-level_peripherals#Internal_Pull-Ups_.26_Pull-Downs
+
+static void init_bcm2835_gpio (void)
+{
+    // Configure "columns" GPIO pins
     GPIO_PULL = 2;  // pull-up
     usleep(1);  // must wait 150 cycles
 #if defined(PCB_SERIAL_MOD_OV) || defined(PCB_SERIAL_MOD_JLW)
     // The Oscar Vermeulen and James L-W serial mods rearrange the PiDP-8/I
     // GPIO matrix to use Pi GPIO pins 2..13, freeing up the hardware
@@ -611,29 +661,52 @@
     GPIO_PULL = 0; // reset GPPUD register
     usleep(1);
     GPIO_PULLCLK0 = 0; // remove clock
     usleep(1); // probably unnecessary
 
-    // BCM2835 ARM Peripherals PDF p 101 & elinux.org/RPi_Low-level_peripherals#Internal_Pull-Ups_.26_Pull-Downs
-    GPIO_PULL = 1;  // pull-down to avoid ghosting (dec2015)
-    usleep(1);  // must wait 150 cycles
-    GPIO_PULLCLK0 = 0x0ff00000; // selects GPIO pins 20..27
-    usleep(1);
-    GPIO_PULL = 0; // reset GPPUD register
-    usleep(1);
-    GPIO_PULLCLK0 = 0; // remove clock
-    usleep(1); // probably unnecessary
-
-    // BCM2835 ARM Peripherals PDF p 101 & elinux.org/RPi_Low-level_peripherals#Internal_Pull-Ups_.26_Pull-Downs
+    // Configure "rows" GPIO pins
     GPIO_PULL = 0;  // no pull-up no pull down just float
     usleep(1);  // must wait 150 cycles
     GPIO_PULLCLK0 = 0x070000; // selects GPIO pins 16..18
     usleep(1);
     GPIO_PULL = 0; // reset GPPUD register
     usleep(1);
     GPIO_PULLCLK0 = 0; // remove clock
     usleep(1); // probably unnecessary
+
+    // Configure "ledrows" GPIO pins
+    GPIO_PULL = 1;  // pull-down to avoid ghosting
+    usleep(1);  // must wait 150 cycles
+    GPIO_PULLCLK0 = 0x0ff00000; // selects GPIO pins 20..27
+    usleep(1);
+    GPIO_PULL = 0; // reset GPPUD register
+    usleep(1);
+    GPIO_PULLCLK0 = 0; // remove clock
+    usleep(1); // probably unnecessary
+}
+
+
+//// init_pidp8i_gpio //////////////////////////////////////////////////
+// Initialize the GPIO pins to the initial states required by
+// gpio_thread().   It's a separate exported function so that scanswitch
+// can also use it.
+
+void init_pidp8i_gpio (void)
+{
+    // Set GPIO pins to their starting state
+    turn_on_pidp8i_leds ();
+    for (size_t i = 0; i < NROWS; i++) {       // Define rows as input
+        INP_GPIO (rows[i]);
+    }
+
+    // Configure GPIO pin pull-ups
+    if (gpio.addr_p == 0xfe200000) {
+        init_bcm2711_gpio();
+    }
+    else {
+        init_bcm2835_gpio();
+    }
 }
 
 
 //// gpio_thread ///////////////////////////////////////////////////////
 // The GPIO thread entry point: initializes GPIO and then calls

Index: src/pidp8i/gpio-common.h
==================================================================
--- src/pidp8i/gpio-common.h
+++ src/pidp8i/gpio-common.h
@@ -1,9 +1,9 @@
 /*
  * gpio-common.h: public interface for the PiDP-8/I's GPIO module
  *
- * Copyright © 2015-2017 Oscar Vermeulen and Warren Young
+ * Copyright © 2015, 2019 by Oscar Vermeulen, © 2016-2019 by Warren Young
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
  * to deal in the Software without restriction, including without limitation
  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
@@ -51,10 +51,16 @@
 #define GPIO_READ(g)  *(pgpio->addr + 13) &= (1<<(g))
 
 #define GPIO_PULL *(pgpio->addr + 37) // pull up/pull down
 #define GPIO_PULLCLK0 *(pgpio->addr + 38) // pull up/pull down clock
 
+// Pi 4 GPIO pull up/down peripheral I/O offsets.  Based on
+// https://github.com/RPi-Distro/raspi-gpio/blob/master/raspi-gpio.c
+#define GPPUPPDN0 57        // pins 15:0
+#define GPPUPPDN1 58        // pins 31:16
+#define GPPUPPDN2 59        // pins 47:32
+#define GPPUPPDN3 60        // pins 57:48
 
 // Switch masks, SSn, used against switchstatus[n]
 #define SS0_SR_B11 04000
 #define SS0_SR_B10 02000
 #define SS0_SR_B09 01000