The analog inputs are read from memory mapped ASIC registers:
6808h ADC0 Analogue Joystick 1, X-Axis (00h=Left=0 ohm, 3Fh=Right=180K 138K ohm) 6809h ADC1 Analogue Joystick 1, Y-Axis (00h=Up =0 ohm, 3Fh=Down =180K 138K ohm) 680Ah ADC2 Analogue Joystick 2, X-Axis (00h=Left=0 ohm, 3Fh=Right=180K 138K ohm) 680Bh ADC3 Analogue Joystick 2, Y-Axis (00h=Up =0 ohm, 3Fh=Down =180K 138K ohm)
680Ch ADC4 Unused, wired to +5V (twice the maximum of 2.5V) (returns 3Fh)
680Dh ADC5 Unused, wired to GND (equivalent to 0 ohm) (returns 00h)
680Fh ADC7 Unused, wired to GND (equivalent to 0 ohm) (returns 00h)
To read that registers: Unlock the ASIC, and then map its register to memory at 4000h..7FFFh.
Values measured by Octoate:
ADC input at 00h..01h = 0 Ohm (left/up)
ADC input at 1Fh..20h = 50 kOhm (center) (that is, the center of ADC values, not of the resistor values)
ADC input at 3Eh..3Fh = 138 kOhm (right/down)
== Analog CPC+ Joysticks ==