The HP67 saves the user program counter in the bottom 3 nibbles (2,1,0) of ram[61]. However, you could be forgiven for thinking that those three digits don’t look anything like the program counter display. Here’s why
\n
\nInternally it stores ram[61][2,1,0] as n,b,r where:
\nn is the instruction number (0-6) within the program register
\nb is the program ram bank (1-2)
\nr is the register (0-0x0f) within the ram bank.<\/p>\n
Program steps 001-112 reside in ram bank 2. You see this as registers 32-47 in the hp67u under menu, Advanced, View RAM.<\/p>\n
Program steps 113-224 reside in ram bank 1. This is registers 16-31 in the same menu option.<\/p>\n
Each ram register contains 14 nibbles (4 bit digits – these show up as “0”-“9” in data registers). When a ram register is used to store a program step, the full range of “0-9a-f” is used for each nibble – so don’t be too surprised if you see things in there other than “0”-“9”.<\/p>\n
Each merged user program step is stored internally as an 8-bit instruction so each program register stores 7 steps and each step is 2 nibbles long.<\/p>\n
This program:<\/p>\n
\r\n001: 31 25 11\r\n002: 09\r\n003: 71\r\n004: 05\r\n005: 81\r\n006: 03\r\n007: 02\r\n008: 61\r\n009: 35 22\r\n010: 84\r\n011: 84\r\n<\/pre>\ngets stored as:<\/p>\n
\r\n45: 00000000000000\r\n46: 00000000000e37\r\n47: 12131e153919fa\r\n<\/pre>\n“fa” is the 8-bit code for “31 25 11” (f LBL A).<\/p>\n
The content of ram[61] looks like this:
\n61: 0000000000042f<\/p>\nThat combination has b=2, r=0x0f(=15) and n=4.<\/p>\n
What you see displayed as the program step number is:
\n((b==2)? 120 : 232) – (r+1)*7 – (n+1)
\nIn this case:
\n=120 – (15+1)*7 – (4+1)
\n=120 – 16*7 – 5
\n=120 – 112 – 5 = 003<\/p>\nEffectively the three n,b,r nibbles form a counter of
\n(0-6)*(1-2)*(0-15) = 7*2*16 = 2*112 = 224 (program steps).<\/p>\nIt’s hard for us to follow but it makes it easier for the calculator to find the next user program step:
\n– select ram bank 1 or 2
\n– access register 0-15 within that bank
\n– get the 0-6th set of 2 “digits” from that register.<\/p>\n","protected":false},"excerpt":{"rendered":"The HP67 saves the user program counter in the bottom 3 nibbles (2,1,0) of ram[61]. However, you could be forgiven for thinking that those three digits don’t look anything like the program counter display. Here’s why<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[16,5,3],"tags":[33],"_links":{"self":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1237"}],"collection":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/comments?post=1237"}],"version-history":[{"count":2,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1237\/revisions"}],"predecessor-version":[{"id":1348,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1237\/revisions\/1348"}],"wp:attachment":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/media?parent=1237"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/categories?post=1237"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/tags?post=1237"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}